Lecture 02: Urogenital System I & II
HoyaDocs — Repro Block | USMLE Step 1 Preparation
Lecture 02 — Overview
Urogenital System I & II · HoyaDocs Repro Block
Page Summaries
Pronephros · Mesonephros · Metanephros · Ureteric bud · Reciprocal induction · Nephron formation · Ascent & rotation
Practice Qs: 1–3
Wolffian (SEED) vs. Müllerian ducts · Urachus · Derivative master table
Practice Qs: 4–6
Practice Qs: 7–9
Practice Qs: 10–13
Practice Qs: 14–18
Official Learning Objectives Map
LO 1 — Kidney Development
Pronephros → Mesonephros → Metanephros · Ureteric Bud · Ascent · HoyaDocs Repro Block
- LO 1: Understand the sequential origin of the three embryologic "kidney" systems and the tissues from which they arise
- LO 2: Correlate the reciprocal induction between the ureteric bud and metanephric blastema with normal kidney and collecting-system architecture
- LO 3: Relate kidney ascent, rotation, and blood-supply turnover to future anatomical/clinical findings on the boards and the wards
▼ 🧫 Embryologic Origins — Why Intermediate Mesoderm Matters
⭐ The entire urogenital tract — kidneys, ureters, gonads, and reproductive ducts — is built from intermediate mesoderm, with a few very testable exceptions.
🧬 The Three Germ Layers (Quick Refresher)
A developing embryo is a tube within a tube made of three stacked layers:
- Ectoderm (outermost) — skin, CNS, adrenal medulla
- Mesoderm (middle) — subdivided into paraxial, intermediate, and lateral plate
- Endoderm (innermost) — gut epithelium, bladder lining, urogenital sinus derivatives
🧭 The Urogenital Ridge
The intermediate mesoderm thickens on either side of the developing aorta to form the urogenital ridge, which runs parallel to the future vertebral column. The ridge has two longitudinal zones:
Nephrogenic cord (lateral) → kidneys, ureters, collecting system
Genital ridge (medial) → gonads (ovaries / testes)
| Urogenital structure | Germ-layer origin | Tested angle |
|---|---|---|
| Kidneys, ureters, gonads, reproductive ducts | Intermediate mesoderm | "UG tract = intermediate mesoderm" stem |
| Adrenal cortex | Mesoderm (coelomic epithelium) | Cortex ≠ medulla origin |
| Adrenal medulla | Ectoderm (neural crest) | Pheo / neuroblastoma arise from neural-crest-derived medulla |
| Bladder (except trigone), urethra | Endoderm (urogenital sinus) | Bladder lining is endodermal |
| External genitalia (penis/labia/clitoris) | Ectoderm (surface) + mesoderm (core) | Skin of external genitalia is ectoderm |
- Cortex = mesoderm; Medulla = ectoderm (neural crest). The adrenal medulla is the odd one out in the urogenital story — it does NOT arise from intermediate mesoderm.
- Because the medulla is neural-crest-derived, pheochromocytoma and neuroblastoma are "neural crest tumors."
- Clinical tie-in: adrenal glands develop separately from kidneys in the upper abdomen. Even with a pelvic kidney, the adrenal stays put.
▼ 🪦 Pronephros (Week 4) — The First, Non-Functional Kidney
⭐ Pronephros appears in the cervical (neck) region at the start of week 4, never forms urine, and regresses by the end of week 4. Its only legacy is the pronephric duct, which will be inherited by the mesonephros.
🔑 Pronephros Essentials
- Location: cervical (neck) nephrogenic cord
- Appearance: beginning of week 4
- Function: NONE — does not make urine
- Fate: regresses by end of week 4 (mesonephros appears before it disappears)
- Legacy: leaves behind the pronephric duct
- The pronephros is non-functional. The mesonephros is the first kidney to make urine (weeks 5–10). The metanephros is the definitive kidney, taking over at week 10.
▼ 🟠 Mesonephros (Weeks 4–10) — Temporary Working Kidney
⭐ The mesonephros appears at the end of week 4 in the thoracolumbar region, actively produces urine from week 5 until week 10, and — critically — donates its duct (the mesonephric / Wolffian duct) to the male reproductive system.
⚙️ Structure & Function
- Located in the thoracic / upper lumbar region
- Contains an S-shaped hollow mesonephric tubule: one end forms a glomerulus-like filter, the other end drains into the mesonephric duct
- The mesonephric duct (derived from the pronephric duct) is long and connects to the cloaca — so the urinary and GI tracts share a common outlet early on
- Actively produces urine between weeks 5 and 10 — this is the first functional "fake" kidney
♂️ Fate of the Mesonephric (Wolffian) Duct
The mesonephric duct is also called the Wolffian duct. Its adult fate is sex-dependent:
♂ With testosterone → rete testis, efferent ductules, epididymis, vas deferens, seminal vesicles, ejaculatory duct + trigone of bladder
♀ No testosterone → regresses (may persist as Gartner duct cyst) · only trigone remains in females
SEED
- S — Seminal vesicles
- E — Epididymis
- E — Ejaculatory duct
- D — Ductus (vas) deferens
- Plus: efferent ductules + rete testis + the trigone of the bladder in both sexes
- The trigone of the bladder is the one piece of bladder wall that comes from the mesonephric ducts (mesoderm), not from the endodermal urogenital sinus. This is why ectopic ureters (which insert below the trigone) behave so strangely — their anatomy follows Wolffian-derived territory.
- Contrast: the rest of the bladder wall is endoderm (urogenital sinus origin).
▼ ⭐ Metanephros & Ureteric Bud (Week 5+) — The Definitive Kidney
⭐ The permanent kidney begins at week 5 as an interaction between two intermediate-mesoderm derivatives: the metanephric blastema (nephron side) and the ureteric bud (plumbing side). Every adult renal structure traces back to one of these two.
🌱 Two Origins, Two Destinies
| Source | Origin | Adult derivatives |
|---|---|---|
| Metanephric blastema (metanephric mesoderm) | Caudal intermediate mesoderm | Nephron: glomerulus / Bowman's capsule, PCT, Loop of Henle, DCT + renal cortex & medulla |
| Ureteric bud | Outgrowth from the distal mesonephric duct | Collecting system: collecting ducts, minor calyces, major calyces, renal pelvis, ureter |
🔁 Reciprocal Induction — How They Find Each Other
The ureteric bud and metanephric blastema each make growth factors that drive the other to grow. This two-way signaling is called reciprocal induction — it is the key concept behind every congenital kidney anomaly.
↓
Ureteric bud grows toward the blastema and secretes WNT9B, FGF2 → induces metanephric mesoderm to condense around it
↓
Week 6 → ureteric stalk lengthens → becomes the ureter
↓
Weeks 7–8 → bud bifurcates: renal pelvis → major calyces → minor calyces → collecting ducts
- Metanephric blastema = NEPHRON (Bowman's capsule → glomerulus → PCT → LOH → DCT) + cortex & medulla.
- Ureteric bud = COLLECTING system (collecting ducts → minor calyces → major calyces → renal pelvis → ureter).
- Failure of reciprocal induction → Renal agenesis (unilateral or bilateral — bilateral causes Potter sequence).
🫗 Nephron Formation (Week 8 onward)
Once the ureteric bud has branched extensively, each collecting-duct tip caps itself in metanephric mesoderm. Cells at the tip induce the surrounding mesoderm to form a metanephric vesicle, which elongates into a comma- then S-shaped body that becomes a single nephron.
S-shaped tubule →
Distal end connects to collecting duct → DCT
Proximal end cups around capillaries → Bowman's capsule + glomerulus + PCT
Middle segment elongates → Loop of Henle
- The definitive metanephros begins producing urine around week 10; by the second trimester, fetal urine is the main source of amniotic fluid.
- Nephrogenesis continues until ~week 36 of gestation — preterm infants may have incomplete nephron counts and increased CKD risk later in life.
- "The ureteric bud forms the nephron" → FALSE. The ureteric bud forms the collecting system only.
- "The metanephric blastema forms the ureter" → FALSE. The ureter comes from the ureteric stalk (a ureteric-bud derivative).
- Memorize the split: Bud = Beyond DCT (collecting), Blastema = Before (and including) DCT (nephron).
▼ 🚀 Kidney Ascent, Rotation & Blood-Supply Switching
⭐ The metanephros starts in the pelvis and ascends to its final lumbar position between weeks 6 and 9, rotating medially as it climbs and switching blood supply along the way.
🧗 Step-by-Step Ascent
- The ureteric bud bifurcates first, so ascent begins before nephrons form.
- As the kidney climbs, it rotates 90° medially — the hilum ends up facing medially (not anteriorly).
- Blood supply is not a single fixed vessel — the kidney acquires new arteries (from iliac → lower → upper abdominal aorta) and the old ones involute.
- Final blood supply: the renal arteries from the abdominal aorta at L1–L2.
- Failure of old vessels to involute → accessory renal arteries (common, usually asymptomatic).
- Kidney fails to ascend → pelvic kidney.
- Midline fusion of metanephric blastemas → horseshoe kidney, which is trapped by the inferior mesenteric artery (IMA) → lies low in the abdomen.
- Because the adrenal glands develop in place near the diaphragm, a pelvic kidney still has an adrenal in the normal position.
- A horseshoe kidney tries to ascend but gets caught under the inferior mesenteric artery. It therefore sits lower than normal (at L3–L5).
- Associated with Turner syndrome (XO), trisomy 18, and increased risk of hydronephrosis, UTIs, kidney stones, and Wilms tumor.
▼ 🗺️ Kidney & Ureter Derivatives — Master Table
⭐ One master table — commit it to memory. This is the single most testable slide in the urinary-tract embryology story.
🧾 Who Makes What
| Embryologic structure | Adult derivative (both sexes) |
|---|---|
| Pronephros | Nothing (regresses); leaves pronephric duct behind |
| Mesonephros (tubules + duct) | Mostly regresses. Duct becomes male internal genitalia (SEED) + trigone in both sexes. |
| Metanephric blastema | Glomerulus, PCT, LOH, DCT — the nephron — plus renal cortex & medulla |
| Ureteric bud | Collecting ducts → minor calyces → major calyces → renal pelvis → ureter |
| Urogenital sinus (upper) | Bladder (except trigone); urethra (females); prostatic + membranous urethra (males) |
| Urogenital sinus (lower / phallic) | Lower 1/3 vagina + vestibule (♀); penile & spongy urethra (♂) |
- Pronephros → Mesonephros → Metanephros = temporal order from week 4 to week 10.
- Pronephros = never functions; Mesonephros = works weeks 5–10; Metanephros = definitive kidney, makes urine from week 10.
- Nephron = metanephric blastema. Collecting system = ureteric bud.
- Ureteric bud arises from the caudal mesonephric duct. No bud → no collecting system → no induction → renal agenesis.
- Kidneys ascend from pelvis to lumbar region, rotating medially. Failure to ascend = pelvic kidney. Midline fusion caught by IMA = horseshoe.
🧠 Testing Your Understanding!
Testing Your Understanding — LO 1, Question 1
A researcher genetically ablates the metanephric blastema in a mouse embryo at gestational day equivalent to human week 5 while leaving the ureteric bud intact. Predict the kidney phenotype at term, and explain why this outcome occurs mechanistically.
Consider: reciprocal induction, GDNF signaling, the role of metanephric mesoderm in nephron formation, what the ureteric bud can build on its own
The metanephric blastema is the source of GDNF, which induces the mesonephric duct to sprout a ureteric bud; reciprocally, it also receives signals (WNT9B, FGF2) from the bud to condense into nephrons. Without the blastema, the bud has no target and no inductive partner — there is no collecting system branching and no nephron formation, so no functional kidney develops. Because amniotic fluid after the second trimester comes from fetal urine, bilateral absence leads to oligohydramnios, fetal compression (flat facies, limb deformities), and pulmonary hypoplasia — the Potter sequence. Consider: how would a unilateral defect differ? (Contralateral compensatory hypertrophy; usually viable.)
Testing Your Understanding — LO 1, Question 2
On fetal ultrasound at 20 weeks, a kidney is noted to sit low in the pelvis. The adrenal gland on the same side appears in its normal superior position. Explain this anatomical dissociation using the embryology of kidney ascent.
Consider: where the kidneys begin, where the adrenals develop, timing of ascent, fate of old blood supply
The metanephros forms in the pelvis and ascends from weeks 6–9, rotating medially and progressively picking up new arterial branches from the aorta as older iliac vessels involute. The adrenal glands develop independently from two sources — the cortex from mesoderm and the medulla from neural-crest ectoderm — near the diaphragm. They do not migrate with the kidney. If the metanephros fails to ascend, the adrenal still takes its normal sub-diaphragmatic position because it was never linked. This explains why a pelvic kidney is typically asymptomatic and incidentally detected. Consider: what blood supply would a pelvic kidney have? (Often variant branches of the common iliac, representing persistence of the kidney's original fetal vessels.)
LO 2 — Bladder, Urethra & Duct Derivatives
Cloaca → Urogenital Sinus · Wolffian vs. Müllerian Duct Fates · HoyaDocs Repro Block
- LO 1: Identify the urinary tract derivatives of the cloaca and urogenital sinus
- LO 2: Correlate Wolffian and Müllerian duct fates with male and female reproductive anatomy
- LO 3: Apply the UG-tract derivatives table to board-style stems and ward anatomy questions
▼ 🕳️ The Cloaca — Common Outlet & Its Split
⭐ Before week 4, the urinary, genital, and GI tracts share one common chamber (the cloaca). A wedge of mesoderm — the urorectal septum — descends and splits the cloaca into an anterior urogenital sinus and a posterior anorectal canal.
⏱️ Timing and Anatomy
↓ urorectal septum grows downward
Week 6–7: septum fuses with the cloacal membrane
↓
Anterior chamber → urogenital sinus (urinary + genital)
Posterior chamber → anorectal canal (GI)
- If the urorectal septum fails to fuse with the cloacal membrane, the urinary and GI tracts remain connected — this is the embryologic basis for rectourethral and rectovaginal fistulas, a high-yield component of the VACTERL association (particularly the "A" = anal atresia + "R" = renal anomalies).
▼ 🏺 Urogenital Sinus — Three Regions, Three Derivatives
⭐ The urogenital sinus is endodermal. It has three regions — upper (vesical), middle (pelvic), and lower (phallic) — each of which becomes different adult structures depending on sex.
📐 Three Regions of the Urogenital Sinus
| Region | In both sexes | Female | Male |
|---|---|---|---|
| Upper (vesical) | → bladder (except trigone) | Bladder | Bladder |
| Middle (pelvic) | → urethra | Entire urethra | Prostatic + membranous urethra; prostate gland buds off; bulbourethral (Cowper's) glands |
| Lower (phallic) | → genital tubercle / vestibule | Lower 1/3 of vagina; vestibule; Bartholin's glands, Skene's glands; hymen | Penile / spongy urethra (after urethral-fold fusion) |
- The bladder is endodermal — except the trigone, which is mesodermal (from absorbed mesonephric ducts).
- In females, the upper 2/3 of vagina is Müllerian (paramesonephric); the lower 1/3 of vagina is UG-sinus (endodermal). The sinovaginal node / vaginal plate is the fusion point.
- The prostate, Skene's glands, and Bartholin's glands are analogous because they all bud off the UG sinus.
▼ 🫧 Bladder & Trigone — Two Tissues, One Organ
⭐ The bladder has a dual origin: the bulk of the wall is endodermal (from the upper urogenital sinus), while the smooth trigone is mesodermal (from absorbed mesonephric ducts).
🧱 Step-by-Step Bladder Assembly
↓
Weeks 5–6 → distal mesonephric ducts (with their ureteric-bud branches) are absorbed into the posterior bladder wall
↓
Absorbed mesonephric tissue → trigone of bladder (smooth, triangular, contains openings of both ureters + internal urethral meatus)
↓
The ureters end up emptying into the bladder; the remaining mesonephric duct enters below in males (to become vas deferens) and regresses in females
The allantois connects the primitive bladder apex to the umbilicus. It normally obliterates to form the median umbilical ligament (urachus). Failure to obliterate → urachal anomalies:
- Patent urachus → urine leaks from umbilicus
- Urachal cyst → fluid-filled midline mass
- Urachal sinus → umbilical drainage without bladder communication
- Urachal remnants can undergo malignant transformation → adenocarcinoma at the bladder dome (classic Step 1 stem)
- The trigone is mesodermal (Wolffian-derived), not endodermal like the rest of the bladder.
- This is why ectopic ureters and other Wolffian-duct abnormalities can produce "trigone pathology" patterns.
- Clinically, most bladder cancers (urothelial/TCC) arise from endodermal urothelium, but adenocarcinoma at the dome points to a urachal (allantois) remnant.
▼ 🛣️ Wolffian vs. Müllerian Ducts — The Two Internal Highways
⭐ Two parallel duct systems form alongside the nephrogenic cord. Which one persists and which one regresses depends entirely on the presence or absence of testicular hormones.
🧭 Duct Anatomy
- Mesonephric (Wolffian) duct — derived from pronephric duct, runs beside the nephrogenic cord, drains into the cloaca/UG sinus. Sprouts the ureteric bud.
- Paramesonephric (Müllerian) duct — forms lateral to the Wolffian duct by invagination of coelomic epithelium. Runs medially; the two paramesonephric ducts fuse in the midline inferiorly.
⚖️ Male vs. Female Duct Fate
The default developmental pathway is female. Male differentiation requires two hormones from the fetal testis:
Sertoli cells → Müllerian Inhibiting Substance (MIS / AMH) → Müllerian duct regresses
Leydig cells → testosterone → Wolffian duct develops into SEED (Seminal vesicles, Epididymis, Ejaculatory duct, Ductus deferens) + efferent ductules + rete testis
Female (no SRY, no hormones):
No MIS → Müllerian duct persists → fallopian tubes, uterus, cervix, upper 2/3 of vagina
No testosterone → Wolffian duct regresses (may persist as Gartner duct cyst on lateral vagina)
| Duct | Hormone required | Source cell | Male adult | Female adult |
|---|---|---|---|---|
| Mesonephric (Wolffian) | Testosterone | Leydig cells | Epididymis, vas deferens, seminal vesicles, ejaculatory duct, efferent ductules, rete testis; trigone | Regresses; trigone; occasional Gartner duct cyst |
| Paramesonephric (Müllerian) | Absence of MIS (regresses if MIS+) | N/A (Sertoli makes MIS) | Regresses; remnants = appendix testis + prostatic utricle | Fallopian tubes, uterus, cervix, upper 2/3 vagina |
- Müllerian agenesis (Mayer–Rokitansky–Küster–Hauser syndrome) — 46,XX with absent upper vagina, uterus, and fallopian tubes; normal ovaries (gonads ≠ ducts); primary amenorrhea with normal secondary sex characteristics.
- Bicornuate uterus / uterine didelphys — incomplete fusion of the two paramesonephric ducts; associated with recurrent miscarriage and renal anomalies (because Wolffian/ureteric bud development is tightly linked to Müllerian development).
- Persistent Müllerian Duct Syndrome (PMDS) — 46,XY male with internal female ducts due to MIS deficiency or MIS-receptor mutation. External genitalia and testosterone effects are normal; classic finding is a uterus/fallopian tubes discovered at surgery for cryptorchidism.
- Bicornuate uterus + ipsilateral renal agenesis = classic "two-hit" embryology question stem.
- Loss of the Y chromosome / SRY gene removes both testosterone and MIS → Müllerian ducts persist (female internal phenotype), Wolffian ducts regress.
- But loss of androgen receptor (as in Complete Androgen Insensitivity) produces a patient with intact testes and MIS — so the Müllerian structures are absent even though the external phenotype is female. Don't confuse the two.
- 5α-reductase deficiency has normal MIS too — internal male ducts (SEED) are present, but external genitalia are female until puberty. The trap is assuming a female phenotype means female internal anatomy.
▼ 🗺️ UG-Tract Derivatives — Master Reference Table
⭐ Memorize this table. Step 1 loves analogies between male and female UG structures because they share an embryologic origin.
🧾 Parts of the Tract → Adult Anatomy
| Embryologic origin | Female | Male |
|---|---|---|
| Ureteric bud | Ureter, renal pelvis, calyces, collecting ducts | Ureter, renal pelvis, calyces, collecting ducts |
| Mesonephric (Wolffian) duct | Trigone of bladder (from absorbed ducts) | Trigone + rete testis, efferent ductules, epididymis, vas deferens, seminal vesicle, ejaculatory duct |
| Paramesonephric (Müllerian) duct | Fallopian tubes, uterus, cervix, upper 2/3 vagina | Regresses (appendix testis, prostatic utricle remnants) |
| Urogenital sinus — upper | Bladder (except trigone) | Bladder (except trigone) |
| Urogenital sinus — middle | Entire urethra; Skene's, Bartholin's glands | Prostatic + membranous urethra; prostate; bulbourethral/Cowper's glands |
| Urogenital sinus — lower (phallic) | Lower 1/3 of vagina; vestibule | Penile / spongy urethra (after urethral-fold fusion) |
| Genital tubercle | Clitoris; glans clitoridis | Glans + shaft of penis |
| Urogenital folds | Labia minora (unfused) | Ventral penile shaft (fuse to form penile urethra) |
| Labioscrotal swellings | Labia majora (unfused) | Scrotum (fuse at midline → raphe) |
| Gubernaculum | Round ligament of uterus + ovarian ligament | Gubernaculum testis (guides testicular descent) |
- Bladder = endoderm (UG sinus) + trigone from mesoderm (Wolffian).
- Wolffian duct = male internal ducts (SEED); needs testosterone.
- Müllerian duct = female internal ducts; regresses in males if Sertoli cells make MIS.
- Upper 2/3 vagina = Müllerian; lower 1/3 vagina + vestibule = UG sinus.
- Analogous male/female pairs: prostate ↔ Skene's; bulbourethral ↔ Bartholin's; penis ↔ clitoris; scrotum ↔ labia majora; labia minora ↔ ventral penile shaft.
🧠 Testing Your Understanding!
Testing Your Understanding — LO 2, Question 1
A 16-year-old phenotypic female presents with primary amenorrhea. Pelvic imaging shows a uterus that appears split into two horns, and her right kidney is absent. Use embryology to explain why these two findings go together.
Consider: Müllerian duct fusion, ureteric bud origin, the shared induction signals between Wolffian and paramesonephric ducts
The two paramesonephric (Müllerian) ducts fuse inferiorly to form the uterus, cervix, and upper 2/3 of the vagina — partial fusion produces a bicornuate uterus. The Müllerian ducts rely on the mesonephric duct as a physical scaffold to guide their descent; this is why paramesonephric and mesonephric development are tightly linked. Because the ureteric bud sprouts from the caudal mesonephric duct, a unilateral Wolffian disruption can simultaneously cause failed ureteric bud formation — producing ipsilateral renal agenesis. Always order a renal ultrasound on any patient with a Müllerian anomaly. Consider: if the defect were bilateral, what would you expect? (Bilateral renal agenesis = Potter sequence.)
Testing Your Understanding — LO 2, Question 2
A newborn boy undergoes exploratory surgery for bilateral non-palpable testes. The surgeon discovers a small uterus and two fallopian tubes adjacent to the inguinal canals. Chromosomal analysis is 46,XY; testosterone, DHT, and gonadotropin levels are all normal. What embryologic mechanism explains this finding?
Consider: Sertoli cells, MIS/AMH, androgen axis, paramesonephric duct regression
In normal male development, Sertoli cells secrete Müllerian Inhibiting Substance (MIS/AMH), which triggers regression of the paramesonephric ducts beginning at week 7–8. Leydig cells concurrently produce testosterone, which drives Wolffian duct development into SEED structures. Because this patient has normal testosterone and normal external male genitalia but retained Müllerian derivatives, the defect is specifically in the MIS axis — either a missense mutation in the AMH gene (MIS deficiency) or in the MIS type II receptor. The retained Müllerian structures often tether the testes intra-abdominally, explaining the associated cryptorchidism. Consider: how does this differ from CAIS? (CAIS has an androgen receptor defect, but MIS is normal — so Müllerian structures are absent.)
LO 3 — Genital Differentiation & External Genitalia
SRY → Gonads → Hormones → External Phenotype · HoyaDocs Repro Block
- LO 1: Understand the hormonal and genetic cascade that converts bipotential gonads and external structures into male or female anatomy
- LO 2: Correlate SRY, testosterone, DHT, and MIS signaling with specific adult internal and external structures
- LO 3: Apply the genital-differentiation cascade to clinical scenarios involving intersex development
▼ 🥚 Gonad Formation — Genital Ridge & Primordial Germ Cells
⭐ The gonads begin as bipotential indifferent gonads on the medial side of the urogenital ridge. Their fate depends on both the genetic sex of the embryo and the arrival of primordial germ cells.
🧭 Sequential Steps
Medial zone of ridge → genital ridge, covered by coelomic epithelium
↓
Primordial germ cells (PGCs) (from endoderm of yolk-sac wall) → migrate along the hindgut dorsal mesentery → seed the genital ridge at week 5
↓
Coelomic epithelium invades mesodermal core → primitive sex cords
↓
Week 6: indifferent (bipotential) gonad is formed; also, paramesonephric (Müllerian) ducts form alongside mesonephric ducts
- No PGCs → no gametes. Somatic gonad can still form a stromal gonad, but the patient will be infertile. Relevant in Turner syndrome (XO), where PGCs arrive but ovarian follicles accelerate into apoptosis — causing "streak gonads."
- Gonadal sex and ductal sex can dissociate (see DSDs tab).
▼ 🧬 The SRY Gene — The Master Switch
⭐ The SRY gene on the short arm of the Y chromosome encodes Testis Determining Factor (TDF). SRY presence → testes. SRY absence → ovaries. Everything else cascades downstream from here.
⚧ SRY Decision Point
Primitive sex cords → Sertoli cells → MIS (AMH) → Müllerian duct regression
Interstitial mesenchyme → Leydig cells (week 8–10) → testosterone → Wolffian duct persistence
Testosterone → 5α-reductase (in target tissue) → DHT → external male genitalia + prostate
SRY absent (46,XX)
Primitive sex cords → ovarian follicles (house oocytes)
No Sertoli cells → no MIS → Müllerian ducts persist (fallopian tubes, uterus, upper vagina)
No Leydig cells → no testosterone → Wolffian ducts regress
No DHT → external female genitalia (clitoris, labia majora, labia minora)
Two-cell model of male differentiation:
- Sertoli → Suppress Müllerian (via MIS)
- Leydig → Look male (via T → DHT)
- Remember: hCG (placental) stimulates fetal Leydig cells until fetal pituitary LH takes over.
- 46,XX male (de la Chapelle syndrome) — SRY has translocated to the X chromosome during paternal meiosis. Patient is 46,XX but phenotypically male with testes. Typically infertile because full spermatogenesis requires other Y-chromosome genes.
- 46,XY female (Swyer syndrome) — deletion or mutation of SRY on the Y. Gonads form as nonfunctional "streak gonads"; no testosterone or MIS → default female internal and external phenotype with primary amenorrhea. Risk of gonadoblastoma → gonadectomy indicated.
▼ 💉 The Hormone Cascade — Who Does What, Where
⭐ Four hormones drive genital differentiation: MIS, testosterone, DHT, and (indirectly) estradiol. Each acts on a specific structure at a specific time.
🧪 Hormone / Enzyme Atlas
| Hormone / enzyme | Made by | Target tissue | Effect |
|---|---|---|---|
| MIS (AMH) | Sertoli cells | Paramesonephric (Müllerian) duct | Induces regression |
| Testosterone | Leydig cells (stimulated by hCG, then LH) | Wolffian duct | Promotes differentiation → SEED structures |
| 5α-reductase | Target tissue (external genitalia, prostate, skin) | Testosterone → DHT | Converts T to potent androgen |
| DHT | (See above) | UG sinus, genital tubercle, urethral/labioscrotal folds, prostate | Virilization of external male genitalia + prostate |
| Aromatase | Gonads, placenta, adipose, skin | Testosterone → estradiol | Estradiol drives female secondary sex characteristics at puberty |
- Testosterone acts on the internal Wolffian ducts directly.
- DHT acts on the external genitalia, prostate, and body hair/acne/sebaceous glands.
- Both signal through the same androgen receptor — a mutation in this receptor (AIS) blocks both downstream actions.
- 5α-reductase deficiency: Wolffian ducts (T-dependent) develop normally, but external genitalia (DHT-dependent) do not masculinize until puberty.
- FALSE — testosterone, not DHT, acts on the Wolffian duct. The Wolffian duct lacks 5α-reductase locally.
- DHT acts on the urogenital sinus (→ prostate, membranous/penile urethra) and the external genitalia (genital tubercle → glans + shaft; urogenital folds → penile raphe; labioscrotal swellings → scrotum).
▼ 🧿 Bipotential External Genitalia (Week 6–7)
⭐ At week 6–7, external genitalia are indistinguishable between XX and XY embryos. Four bipotential structures surround the cloacal membrane and will either fuse (male) or remain open (female).
🗿 The Four Bipotential Structures
| Bipotential structure | With DHT (♂) | Without DHT (♀) |
|---|---|---|
| Genital tubercle | Elongates → glans & shaft of penis | Stays small → clitoris |
| Urogenital (urethral) folds | Fuse along midline → ventral penile shaft + penile urethra | Remain unfused → labia minora |
| Labioscrotal swellings | Fuse at midline → scrotum (raphe = fusion seam) | Remain unfused → labia majora |
| Urogenital sinus (phallic portion) | Penile/spongy urethra, prostate | Lower 1/3 vagina, vestibule, Skene's & Bartholin's glands |
DHT required for: elongation of genital tubercle + fusion of urogenital folds + fusion of labioscrotal swellings.
Without DHT at any of these three steps: the external phenotype is female or ambiguous.
♂️ Male External Differentiation (DHT Pathway)
- Testosterone is converted to DHT by 5α-reductase in the skin/fascia of the external genitalia.
- DHT drives: genital tubercle elongation → penis; urogenital folds fuse ventrally → penile urethra (meatus extends to glans tip via ingrowth of surface ectoderm); labioscrotal swellings fuse at midline → scrotum (raphe is the visible midline seam).
- A small ectodermal invagination from the glans tip helps complete the external urethral meatus.
♀️ Female External Differentiation (Default / No DHT)
- No DHT → no fusion. The genital tubercle bends forward and becomes the clitoris.
- Urogenital folds remain unfused → labia minora; labioscrotal swellings remain unfused → labia majora.
- The urethra and vagina open separately into the urogenital sinus → vestibule.
- Androgen exposure to a genetic female in the first trimester (e.g., CAH, maternal androgenic tumor, exogenous androgens) → labioscrotal fusion + clitoromegaly + common urogenital sinus = genital virilization.
- Androgen exposure in the third trimester only causes clitoromegaly, not fusion (the labioscrotal folds have already decided their fate). Timing matters!
▼ 🧾 Putting It All Together — Internal + External Cheat Sheet
⭐ One table to unify gonadal sex, ductal sex, and phenotypic sex. Use it to predict the consequence of any single hormonal lesion.
🧩 If X is missing, predict the phenotype
| Missing component | Gonad | Internal ducts | External genitalia |
|---|---|---|---|
| Normal male (reference) | Testes | Wolffian (SEED); Müllerian absent | Penis, scrotum |
| Normal female (reference) | Ovaries | Müllerian (tubes, uterus, upper vagina); Wolffian absent | Clitoris, labia, lower 1/3 vagina |
| SRY missing (46,XY Swyer) | Streak | Müllerian (no MIS, no T) | Female |
| Sertoli/MIS missing (PMDS) | Testes | Both Wolffian + Müllerian | Male (T present) |
| Leydig/T missing | Testes | Müllerian only (MIS works) | Female |
| 5α-reductase deficiency | Testes | Wolffian present (T-dependent) | Female or ambiguous at birth; virilizes at puberty |
| Androgen receptor mutation (CAIS) | Testes | Neither present (MIS works; T can't signal) | Female (short blind vagina) |
| First-trimester androgen excess in XX (CAH) | Ovaries | Müllerian present (normal) | Virilized / ambiguous |
- SRY on Y → testes. Without SRY → default ovaries.
- MIS from Sertoli → Müllerian regression.
- T from Leydig → Wolffian persistence (internal male).
- DHT (via 5α-reductase) → external virilization + prostate.
- Aromatase converts T to estradiol — relevant when testes are retained in CAIS (estrogen drives breast development).
🧠 Testing Your Understanding!
Testing Your Understanding — LO 3, Question 1
A 14-year-old with 46,XY karyotype presents with primary amenorrhea. On exam, breast development is Tanner IV, pubic and axillary hair are sparse, and the vagina is short and blind-ending. Uterus and fallopian tubes are absent on imaging; bilateral inguinal masses are palpable. Map this phenotype to the embryology: which hormones are working, which are not?
Consider: SRY, MIS, testosterone, DHT, androgen receptor, aromatase
The SRY gene is active → testes form → both Sertoli (MIS works → Müllerian regression → no uterus/upper vagina) and Leydig (testosterone normal or high) are functional. However, the androgen receptor is nonfunctional, so neither testosterone nor DHT can signal in target tissues: Wolffian ducts regress (no SEED), external genitalia default to female (no DHT effect), and body hair fails at puberty. Aromatase converts circulating testosterone to estradiol, driving breast development at puberty. The "inguinal masses" are the undescended testes — they must be removed after puberty due to malignancy risk. Consider: how would partial androgen insensitivity differ? (Ambiguous external genitalia; some virilization at puberty.)
Testing Your Understanding — LO 3, Question 2
A 46,XY fetus has a selective block that prevents conversion of testosterone to DHT. Predict the internal ductal anatomy and external genitalia at birth, and explain what changes at puberty and why.
Consider: Wolffian duct ligand, external genitalia ligand, pubertal testosterone surge, source of virilization
The Wolffian duct is testosterone-dependent, so internal male ducts (SEED) are preserved. Sertoli cells still make MIS, so Müllerian ducts regress as expected. However, the external genitalia and prostate require DHT — without it, the genital tubercle doesn't elongate, the urogenital folds don't fuse, and the labioscrotal swellings don't close. Phenotype at birth: ambiguous or frankly female external genitalia, often with penoscrotal hypospadias, a blind-ending vagina, and labial fusion. At puberty, the massive testosterone surge can directly drive some virilization (deepening voice, muscle mass, phallic growth) via high testosterone-to-androgen-receptor signaling in tissues where T alone is sufficient. Lab finding: high testosterone-to-DHT ratio. Consider: how would this differ from CAIS? (In CAIS, the androgen receptor itself is broken, so puberty cannot drive virilization.)
LO 4 — Urinary & Genital Anomalies
Renal malformations · Ureter defects · Testicular descent · Hypospadias/Epispadias · HoyaDocs Repro Block
- LO 1: Recognize common congenital urogenital anomalies and their clinical presentations
- LO 2: Correlate each anomaly with a specific developmental error (ureteric bud, kidney ascent, urethral fold fusion, gubernaculum)
- LO 3: Apply this to board-style vignettes and ward imaging findings
- Potter Sequence & Renal Agenesis
- Multicystic Dysplastic Kidney (MCDK)
- Horseshoe Kidney
- Pelvic (Ectopic) Kidney
- Ureteric-Bud Anomalies
- ↳ Ureteral Duplication
- ↳ Ectopic Ureter
- ↳ Uretero-Pelvic Junction Obstruction, Megaureter, VUR, Ureterocele
- Testicular Descent & Cryptorchidism
- Hypospadias & Epispadias
▼ 💀 Potter Sequence — When Bilateral Renal Function Fails
⭐ Whenever the fetus cannot make or excrete urine, amniotic fluid drops (oligohydramnios). Low amniotic fluid compresses the fetus and starves the lungs of the distension they need — producing the classic Potter phenotype.
🧬 Causes
- Bilateral Renal Agenesis (BRA) — ureteric bud or metanephric blastema fails to form on both sides; no kidneys on imaging
- Autosomal Recessive Polycystic Kidney Disease (ARPKD)
- Bilateral obstructive uropathy — e.g., posterior urethral valves (PUV)
- Esophageal atresia or other upstream issues preventing the fetus from swallowing amniotic fluid (VACTERL association)
👶 The Classic Potter Phenotype
↓
Mechanical fetal compression → flattened facies + clubbed feet + limb/joint deformities
↓
No amniotic-fluid-driven lung distention → pulmonary hypoplasia (the actual cause of death)
POTTER
- P — Pulmonary hypoplasia (→ respiratory failure; cause of death)
- O — Oligohydramnios (trigger)
- T — Twisted face
- T — Twisted skin/extremities
- E — Extremity defects
- R — Renal failure (in utero)
- Cause of death is pulmonary hypoplasia → respiratory failure, not kidney failure. The placenta handles fetal waste during gestation; the lungs don't get a second chance.
▼ 🎈 Multicystic Dysplastic Kidney (MCDK)
⭐ Abnormal interaction between the ureteric bud and metanephric blastema → normal renal tissue is replaced by non-functioning cysts of various sizes. Usually unilateral and compatible with life.
Key Features
- Pathogenesis: failed reciprocal induction → abnormal collecting system + dysplastic nephrons → replacement of parenchyma with cysts
- Laterality: usually unilateral (compatible with life); bilateral MCDK is incompatible with life (causes Potter sequence)
- Imaging: "bunch of grapes" appearance on prenatal ultrasound; non-functional kidney
- Associations: most common cause of abdominal mass in a newborn; contralateral hydronephrosis or VUR in ~30%
- MCDK — embryologic maldevelopment, usually unilateral, not inherited.
- ARPKD — PKHD1 mutation, bilateral enlarged kidneys with collecting-duct cysts, presents in infancy with Potter sequence, hepatic fibrosis.
- ADPKD — PKD1/PKD2, adult-onset HTN, massive bilateral kidneys, berry aneurysms, hepatic cysts, MVP.
▼ 🐴 Horseshoe Kidney
⭐ Lower poles of the two metanephric blastemas fuse across the midline → a U-shaped single kidney that ascends until it is blocked by the inferior mesenteric artery (IMA) at L3–L5.
Clinical Pearls
- Typically found incidentally on prenatal ultrasound, CT, MRI, or IVP
- Most individuals are asymptomatic; some have increased risk of:
- UTIs and vesicoureteral reflux
- Kidney stones
- Hydronephrosis
- Wilms tumor
- Renal cell carcinoma
- Associations: Turner syndrome (XO), trisomy 13, 18, 21
- The fused isthmus is caught on the inferior mesenteric artery as the kidney tries to ascend normally. Memorize: horseshoe kidney = stopped at the IMA.
▼ ⬇️ Pelvic (Ectopic) Kidney
⭐ One kidney fails to ascend from its pelvic origin to the lumbar region. The adrenal gland is in its normal position (adrenals do not migrate with the kidneys).
Key Features
- Usually asymptomatic, found incidentally on imaging
- Blood supply often abnormal — accessory/variant vessels from the iliac or aorta (persistence of old fetal arterial supply)
- Predisposes to UTIs, stones, and reflux due to altered anatomy
- Adrenal gland on the affected side sits in normal position under the diaphragm
▼ 🔀 Ureteric-Bud & Ureter Anomalies
⭐ Problems with the ureteric bud at any stage — too many buds, wrong position, abnormal insertion, or wall defects — produce a cluster of related congenital anomalies.
▼Ureteral Duplication
- Two ureters drain a single kidney — from premature bifurcation of the ureteric bud.
- Complete duplication: two ureters reach the bladder separately.
- Incomplete (bifid) duplication: two ureters from separate renal pelvises join proximally before inserting into the bladder.
- Clinical: most common renal structural anomaly; can predispose to VUR and infection, especially in the upper-pole ureter.
▼Ectopic Ureter
- Ureter inserts outside the normal trigone — because the mesonephric duct was abnormally absorbed into the bladder wall.
- Often associated with a duplex collecting system (classically drains the upper pole).
- Females: the ectopic ureter often drains below the external urethral sphincter (into vagina, vestibule, or urethra distal to the sphincter) → continuous urinary incontinence despite normal voiding from the other, normally inserting ureter.
- Males: can drain into mesonephric derivatives (vas deferens, seminal vesicle, epididymis) — but always above the external sphincter, so no constant leakage.
▼UPJ Obstruction · Megaureter · VUR · Ureterocele
- Uretero-Pelvic Junction (UPJ) Obstruction — most common cause of fetal hydronephrosis; blockage where renal pelvis meets the ureter.
- Megaureter — dilated ureter. Can be due to reflux, obstruction, or a non-obstructive wall-muscle deficiency (weak ureteric wall can't push urine).
- Vesico-Ureteral Reflux (VUR) — retrograde flow from bladder into ureters/kidneys through a faulty ureterovesical junction valve. Predisposes to recurrent UTIs, renal scarring, and reflux nephropathy. Graded I (into ureter only) → V (severe hydroureteronephrosis + tortuosity).
- Ureterocele — balloon-like cystic dilation of the distal intramural ureter. Classic "cobra-head sign" on imaging. Often associated with duplex kidney (with the ectopic ureter's insertion).
- In females, the distal ureter can open below the external sphincter (urethra, vestibule, vagina) → constant dribbling.
- In males, any ectopic insertion is still above the external sphincter (vas deferens, seminal vesicle) → no continuous leakage, but risk of epididymitis or fertility issues.
▼ ⚽ Testicular Descent & Cryptorchidism
⭐ The testes begin in the abdomen at the level of T10 and descend into the scrotum guided by the gubernaculum. Each layer they pass through becomes a layer of the adult spermatic cord.
🧭 Path of Descent
↓
Months 7–8: testis descends through the retroperitoneum → inguinal canal → via the processus vaginalis (peritoneal outpouching) into the scrotum
↓
After descent: processus vaginalis should obliterate to seal the scrotum from the abdomen
The remaining scrotal portion becomes the tunica vaginalis around the testis
Layers Derived from the Abdominal Wall
| Abdominal-wall layer | Spermatic-cord layer |
|---|---|
| External oblique aponeurosis | External spermatic fascia |
| Internal oblique | Cremaster muscle + cremasteric fascia |
| Transversalis fascia | Internal spermatic fascia |
🎯 Cryptorchidism
- Definition: failure of one or both testes to descend into the scrotum. Most commonly, the testis is found in the inguinal canal.
- Associations: deficiencies of gonadotropins or androgens; disorders of sexual differentiation (DSD); Kallmann syndrome; Prader-Willi.
- Natural history: many spontaneously descend by 6 months. If not, orchidopexy is performed — the earlier the better, ideally before 1 year.
- Consequences if uncorrected:
- ↓ Fertility (heat damages seminiferous tubules; reduced spermatogenesis)
- ↑ Risk of germ cell tumors (seminoma most common in an abdominal testis) — early orchidopexy reduces malignancy risk
- Associated indirect inguinal hernia (patent processus vaginalis) & testicular torsion
- Labs: testosterone levels are typically normal (Leydig cells intact); inhibin B is low (Sertoli damage from heat) and FSH is elevated.
💧 Processus Vaginalis Failures
- Patent processus vaginalis (PPV) → indirect inguinal hernia or communicating hydrocele (fluid flows between abdomen and scrotum).
- Partial obliteration with a trapped segment → non-communicating hydrocele or spermatic cord cyst.
- Hydrocele classic finding: transilluminates; indirect hernia does not.
- Leydig cells are relatively heat-resistant → testosterone is usually preserved.
- Seminiferous tubules (Sertoli cells + germ cells) are heat-sensitive → ↓ inhibin B, ↑ FSH, ↓ sperm count. This is the classic lab pattern on Step 1.
▼ 🕳️ Hypospadias & Epispadias
⭐ Failure of urethral fold fusion along the ventral penis → hypospadias. Abnormal genital tubercle positioning → epispadias. Remember: hypo is on the underside, epi is on top.
Hypospadias
- Mechanism: failure of the urogenital (urethral) folds to fuse ventrally along the shaft → urethral opening is on the ventral (inferior) surface of the penis.
- Location: anywhere along the penile raphe; can be glanular, penile, penoscrotal, or perineal. May have multiple openings.
- More common than epispadias.
- Associations: cryptorchidism (shared androgen-dependent development); DSDs (e.g., 5α-reductase deficiency → often penoscrotal hypospadias).
- Treatment: urethroplasty (often with buccal mucosa graft).
- Caution: do not circumcise — foreskin is used for surgical repair.
Epispadias
- Mechanism: abnormally positioned genital tubercle / defective cloacal membrane → urethra opens on the dorsal (superior) surface.
- Less common than hypospadias.
- Associated with bladder exstrophy (failure of abdominal wall closure exposes bladder mucosa).
- Hypo = below / ventral = urethral fold fusion failure.
- Epi = above / dorsal = genital tubercle / cloacal membrane defect; associated with bladder exstrophy.
- Do not circumcise hypospadias — the foreskin is used for repair.
🧠 Testing Your Understanding!
Testing Your Understanding — LO 4, Question 1
A 4-year-old girl is referred for evaluation of "always being a little wet." Her mother reports that she voids normally every 2–3 hours, but there is continuous dribbling of urine between voids. Renal ultrasound shows a duplex collecting system on the right. Explain the embryologic mechanism and predict the exact anatomic location of the abnormal ureter.
Consider: ureteric bud absorption into the bladder, upper vs. lower pole ureter in duplex kidneys, location of the external urethral sphincter in females
With a duplex kidney, two ureters exist. By the Weigert-Meyer rule, the upper-pole ureter inserts inferiorly and medially (often ectopically) while the lower-pole ureter inserts normally at the trigone. An ectopic ureter results from abnormal absorption of the mesonephric duct into the bladder wall — the ectopic opening ends up outside the normal trigone. In females, this is often distal to the external urethral sphincter (urethra, vagina, or vestibule), which bypasses voluntary continence → constant dribbling despite normal voiding through the other ureter. In males, ectopic ureters open above the sphincter, so continuous incontinence doesn't occur. Consider: what additional finding often accompanies this? (Ureterocele of the upper-pole ureter with a "cobra-head" sign on imaging.)
Testing Your Understanding — LO 4, Question 2
A newborn boy is noted to have a small scrotum with only a left palpable testis, and a urethral opening visible on the ventral surface of the penis halfway down the shaft. Map the embryology: which developmental event produced each finding, and what single hormonal axis is implicated in both?
Consider: testicular descent, gubernaculum, DHT action on external genitalia, urethral fold fusion
Testicular descent through the inguinal canal is guided by the gubernaculum and requires normal androgen signaling (particularly in the second descent phase). Hypospadias results from failure of the urogenital (urethral) folds to fuse along the ventral penis — a process driven by DHT. When both defects co-occur, suspect either a mild DSD (partial androgen insensitivity, 5α-reductase deficiency) or a maternal/placental cause of relative androgen deficiency. Important management pearls: (1) do NOT circumcise a newborn with hypospadias — the foreskin is reserved for urethroplasty; (2) if testes are non-palpable bilaterally, order a karyotype to rule out a virilized 46,XX with CAH — giving testosterone to a virilized female could cause harm. Consider: what is the most common histologic finding in a retained abdominal testis after age 10? (Seminoma precursor, GCNIS → orchidopexy before age 1.)
LO 5 — Disorders of Sex Development (DSDs)
CAH · Androgen Insensitivity · 5α-Reductase · Klinefelter · Turner · HoyaDocs Repro Block
- LO 1: Understand the adrenal steroidogenesis axis and how enzyme blocks shunt precursors into androgens
- LO 2: Correlate each DSD with karyotype, hormonal profile, internal and external phenotype
- LO 3: Distinguish and apply chromosomal DSDs (Klinefelter, Turner) from hormonal DSDs on boards-style vignettes
- Adrenal Cortex & Steroidogenesis Refresher
- Congenital Adrenal Hyperplasia (CAH)
- ↳ 21-Hydroxylase Deficiency (95%)
- ↳ 11β-Hydroxylase Deficiency
- ↳ 17α-Hydroxylase Deficiency
- ↳ 3β-Hydroxysteroid Dehydrogenase
- Androgen Insensitivity Syndrome (CAIS / PAIS)
- 5α-Reductase Deficiency
- Other 46,XX Virilization Causes
- Klinefelter Syndrome (47,XXY)
- Turner Syndrome (45,XO)
- DSD Summary Chart
▼ 🏛️ Adrenal Cortex Refresher — Why CAH Virilizes
⭐ The adrenal cortex makes three hormone classes — mineralocorticoids, glucocorticoids, and sex steroids — from a shared cholesterol backbone. A block in one pathway shunts precursors into whichever pathway is still open.
🧭 Zones of the Adrenal Cortex
GFR from outer → inner cortex:
- Glomerulosa → Salty → mineralocorticoids (aldosterone). Regulated by angiotensin II/K⁺.
- Fasciculata → Sweet → glucocorticoids (cortisol). Regulated by ACTH.
- Reticularis → Sexy → androgens (DHEA, androstenedione). Regulated by ACTH.
- The medulla (ectoderm/neural crest) → catecholamines.
↓ 20,22-desmolase (StAR, rate-limiting; ACTH-stimulated)
Pregnenolone
→ 3β-HSD → progesterone
→ 21-hydroxylase → 11-deoxycorticosterone
→ 11β-hydroxylase → corticosterone / aldosterone
Parallel glucocorticoid branch: 17-hydroxyprogesterone → 11-deoxycortisol → cortisol
Parallel androgen branch: 17α-hydroxylase + 17,20-lyase → DHEA → androstenedione → testosterone → estradiol (via aromatase)
▼ 🧂 Congenital Adrenal Hyperplasia (CAH)
⭐ CAH is a family of autosomal recessive enzyme deficiencies in adrenal steroidogenesis. A partial cortisol deficit removes negative feedback → ↑ ACTH → adrenal hyperplasia and diversion of precursors into whichever pathway(s) are still open.
▼21-Hydroxylase Deficiency (95% of CAH)
- Block: cannot make 11-deoxycorticosterone or 11-deoxycortisol → ↓ aldosterone, ↓ cortisol.
- Shunt: progesterone & 17-OH progesterone accumulate → pushed into androgens → ↑↑ DHEA, androstenedione, testosterone, DHT.
- Phenotype: 46,XX infants are virilized (ambiguous genitalia); 46,XY infants look normal at birth but present with salt-wasting crisis (hyponatremia, hyperkalemia, hypotension, vomiting) in the first 1–2 weeks of life.
- Labs: ↑ 17-OH progesterone (pathognomonic screening lab), ↑ renin, ↓ aldosterone, ↓ cortisol, ↑ ACTH, hyponatremia, hyperkalemia, metabolic acidosis, hypoglycemia.
- Treatment: lifelong glucocorticoid replacement (hydrocortisone) — this replaces cortisol and suppresses ACTH → reduces shunting. Add fludrocortisone if salt-wasting. Feminizing genitoplasty at 3–6 months for virilized females.
▼11β-Hydroxylase Deficiency
- Block: cannot convert 11-deoxycorticosterone → aldosterone, or 11-deoxycortisol → cortisol.
- Key feature: 11-deoxycorticosterone (DOC) accumulates and is itself a potent mineralocorticoid → hypertension, low renin, hypokalemia.
- Androgens: still elevated → virilization of 46,XX infants.
- Buzzwords: "virilized female with HTN" = 11β-hydroxylase.
▼17α-Hydroxylase Deficiency (no sex hormones + HTN)
- Block: cannot synthesize cortisol or sex steroids. Mineralocorticoid pathway is unaffected → DOC and corticosterone accumulate.
- Phenotype: HTN + hypokalemia + hypogonadism. 46,XY patients present as phenotypic females (no testosterone → no virilization); 46,XX patients fail to enter puberty.
- Buzzwords: "tall phenotypic female with HTN, no breast development, primary amenorrhea" = 17α-hydroxylase.
▼3β-Hydroxysteroid Dehydrogenase Deficiency
- Block: upstream enzyme affecting all three pathways → deficient aldosterone, cortisol, and potent androgens.
- DHEA accumulates (weak androgen) — can mildly virilize XX females and under-virilize XY males (ambiguous genitalia in both).
- Clinical: salt-wasting similar to 21-OH deficiency; severe form is rare.
🧾 CAH Cheat Sheet
| Enzyme | Aldosterone | Cortisol | Androgens | BP | Genitalia (46,XX) | Genitalia (46,XY) |
|---|---|---|---|---|---|---|
| 21-OH | ↓ | ↓ | ↑↑ | ↓ (salt-wasting) | Virilized / ambiguous | Normal male |
| 11β-OH | ↓ (but DOC↑ so HTN) | ↓ | ↑↑ | ↑ HTN, ↓ K⁺ | Virilized | Normal male |
| 17α-OH | ↑ (DOC) | ↓ | ↓↓ | ↑ HTN, ↓ K⁺ | Phenotypic female, primary amenorrhea | Phenotypic female, primary amenorrhea |
| 3β-HSD | ↓ | ↓ | ↓ potent / ↑ weak (DHEA) | ↓ salt-wasting | Mild virilization (DHEA) | Under-virilized / ambiguous |
- Proximal blocks (3β-HSD, 21-OH) → salt-wasting / hypotension (no mineralocorticoid path).
- Distal blocks that spare DOC (11β-OH, 17α-OH) → hypertension from accumulated DOC.
- Rule of thumb: if BP is high, think "one-one" (11β or 17α); if BP is low, think "two-one" (21-OH).
- The newborn screen measures 17-OH progesterone, which is specifically elevated in 21-hydroxylase deficiency.
- Monitor treatment response with 17-OH progesterone levels — if still elevated on steroids, ACTH suppression is inadequate.
- In 46,XX infants with ambiguous genitalia + salt-wasting, do NOT give testosterone. Confirm karyotype, measure 17-OH progesterone, electrolytes, and give hydrocortisone.
▼ 🧬 Androgen Insensitivity Syndrome (AIS / CAIS / PAIS)
⭐ Most common cause of 46,XY DSD. X-linked recessive mutation in the androgen receptor (AR) → tissues cannot respond to testosterone or DHT. SRY, Sertoli, Leydig all functional; MIS works normally.
Complete Androgen Insensitivity (CAIS)
- Karyotype: 46,XY
- Gonads: bilateral testes (often intra-abdominal or inguinal)
- Internal ducts: Müllerian ducts absent (MIS works); Wolffian ducts absent (AR nonfunctional) → short, blind-ending vagina, no uterus/fallopian tubes
- External genitalia: normal female
- Hormones: testosterone and LH are normal-to-high (no feedback suppression), DHT normal, gonadotropins eventually rise → aromatase-converted estradiol drives breast development at puberty
- Classic presentation: tall adolescent girl with breast development, primary amenorrhea, sparse/absent pubic and axillary hair, inguinal masses
- Management: raise as female, estrogen therapy, gonadectomy after puberty (testes allow pubertal feminization via aromatase; remove to prevent malignancy)
Partial Androgen Insensitivity (PAIS)
- Spectrum from mild undervirilization (hypospadias, gynecomastia, infertility) to ambiguous genitalia.
- Management decisions depend on severity; often individualized.
- The testes are present and functional → Sertoli cells still make MIS, which causes Müllerian duct regression. So no uterus or fallopian tubes — despite the female external phenotype.
- Contrast: in Müllerian agenesis (MRKH, 46,XX), ovaries are normal but Müllerian ducts never form. Both present with primary amenorrhea and blind vagina, but karyotype distinguishes them.
▼ 🚫 5α-Reductase Deficiency
⭐ Autosomal recessive inability to convert testosterone to DHT in the external genitalia and prostate. Internal ducts (T-dependent) form normally; external genitalia (DHT-dependent) do not — until the pubertal testosterone surge.
Key Features
- Karyotype: 46,XY
- Internal ducts: normal Wolffian structures (SEED) — testosterone works; no Müllerian structures — MIS works
- External genitalia at birth: female or ambiguous — small phallus ("enlarged clitoris"), penoscrotal hypospadias, labial fusion, blind-ending vagina
- At puberty: massive testosterone rise overwhelms the block → virilization (deepening voice, muscle mass, phallic growth) — classic "female at birth, male at puberty" presentation
- Labs: ↑ testosterone:DHT ratio; LH normal-high
- No gynecomastia (unlike CAIS — because androgen receptor works, so T still suppresses aromatase-related breast tissue expansion)
- CAIS: 46,XY, female external phenotype, breast development at puberty, sparse hair, no virilization.
- 5α-reductase: 46,XY, female/ambiguous at birth, virilization at puberty with deep voice & phallic growth, but no breast development.
- Swyer (46,XY gonadal dysgenesis): 46,XY, streak gonads, no testosterone → female phenotype + no pubertal virilization or breast development (needs estrogen replacement).
▼ 👩🦰 46,XX Externally Virilized — Non-CAH Causes
⭐ Virilization of a genetically female fetus from maternal or placental androgen exposure. Timing (first trimester vs. third trimester) determines whether there is labioscrotal fusion or only clitoromegaly.
Causes
- Maternal androgen-secreting tumors (e.g., luteoma, Krukenberg, adrenal adenoma)
- Iatrogenic androgens — danazol for endometriosis, testosterone supplementation, anabolic steroid use in pregnancy
- Progestins with androgenic activity
- Placental aromatase deficiency — can't convert fetal adrenal precursors (DHEA-S) to estradiol → fetal testosterone rises → virilization of the fetus AND maternal virilization during pregnancy
- SRY translocation (46,XX male / de la Chapelle syndrome) — testes form, testosterone virilizes
▼ ♂️ Klinefelter Syndrome — 47,XXY
⭐ Most common cause of male hypogonadism and of infertility in men. Nondisjunction in meiosis → at least one Y + at least two X chromosomes → dysgenesis of seminiferous tubules.
Features
- Internal ducts: SRY present → testes + MIS → Müllerian regression. Testosterone present → male Wolffian derivatives (SEED).
- Testes: small, firm, fibrotic, hyalinized seminiferous tubules → infertility (azoospermia); Leydig cells predominate and are dysfunctional.
- Hormones:
- ↓ Inhibin B (Sertoli dysfunction)
- ↓ or low-normal testosterone
- ↑↑ FSH (loss of inhibin B feedback), ↑ LH (partial Leydig failure)
- ↑ Estradiol (hCG/LH stimulating residual aromatase) → gynecomastia, female fat distribution, tall stature with eunuchoid proportions
- Phenotype: tall, long extremities, small testes, gynecomastia, developmental delay/learning difficulty
- Malignancy risk: ↑ breast cancer (vs. other men), extragonadal germ cell tumors (mediastinal)
- Hallmark exam finding: very small, firm testes in an otherwise tall man
- Mosaic 46,XY/47,XXY can have milder presentation or even preserved fertility — karyotype should be reported with cell-line distribution.
▼ ♀️ Turner Syndrome — 45,XO
⭐ Partial or complete loss of one X chromosome (often paternal origin) → ovarian dysgenesis with accelerated oocyte apoptosis → "streak gonads." Most common cause of primary amenorrhea.
Features
- Gonads: streak ovaries — no follicles (accelerated atresia); no estrogen → no secondary sex characteristics
- Internal + external genitalia: female (default pathway, no androgens, no MIS)
- Hormones: ↓ estrogen, ↑↑ FSH, ↑ LH ("hypergonadotropic hypogonadism")
- Classic physical features: short stature, webbed neck, cystic hygroma (prenatal), shield chest with widely-spaced nipples, cubitus valgus, lymphedema of the hands/feet in infancy, low posterior hairline, ovarian streaks
- Associated anomalies:
- Cardiovascular: bicuspid aortic valve (most common); coarctation of the aorta (classic stem); increased aortic dissection risk
- Renal: horseshoe kidney (major repro-module overlap)
- Endocrine: Hashimoto's thyroiditis, type 1 and type 2 diabetes
- Lymphatic: cystic hygroma, pedal edema
- Management: growth hormone for short stature; estrogen replacement starting around puberty (induces secondary sex characteristics); monitor for cardiac/renal complications; fertility is rare — donor oocytes may be needed.
- ~75% of Turner patients have lost the paternal X (paternal nondisjunction is more common). The retained X is almost always maternal.
- Even with one X, haploinsufficiency of SHOX and other X-encoded genes drives the phenotype.
▼ 🧾 DSD Summary — One Table, Everything You Need
⭐ Cross-reference karyotype, gonad, internal/external phenotype, and labs. Step 1 loves pattern recognition here.
Comparison Chart
| Condition | Karyotype | Gonad | Internal | External | Key Lab |
|---|---|---|---|---|---|
| 21-OH CAH | 46,XX | Ovaries | Normal Müllerian | Virilized | ↑↑ 17-OH progesterone; ↓ Na⁺, ↑ K⁺ |
| 11β-OH CAH | 46,XX | Ovaries | Normal Müllerian | Virilized | ↑ DOC; HTN, ↓ K⁺ |
| 17α-OH CAH | 46,XX or XY | Ovaries or Testes | Varies | Female / ambiguous | ↑ DOC, ↓ sex steroids; HTN, ↓ K⁺ |
| CAIS | 46,XY | Testes (inguinal) | No Müllerian, no Wolffian | Female, short blind vagina | ↑ T, ↑ LH; breast dev from estradiol |
| 5α-reductase | 46,XY | Testes | Wolffian present; Müllerian absent | Female/ambiguous at birth → male at puberty | ↑ T:DHT ratio |
| Swyer (46,XY gonadal dysgenesis) | 46,XY | Streak | Müllerian (no MIS) | Female | ↑ FSH, ↓ T, ↓ estradiol |
| Klinefelter | 47,XXY | Dysgenic testes | Wolffian (normal male) | Tall male, gynecomastia, small testes | ↑ FSH, ↑ LH, ↓ inhibin B, ↓–normal T, ↑ estradiol |
| Turner | 45,XO | Streak ovaries | Normal female | Female + webbed neck, short stature | ↑ FSH, ↓ estrogen; horseshoe kidney, bicuspid aortic valve |
| 46,XX male (SRY translocation) | 46,XX | Testes | Wolffian | Male (infertile) | SRY on X; azoospermia |
| MRKH (Müllerian agenesis) | 46,XX | Ovaries | No Müllerian | Female; blind vagina, no uterus | Normal T/estradiol; karyotype distinguishes from CAIS |
- CAH = enzyme name determines phenotype. 21-OH = salt-wasting + virilization; 11β-OH = HTN + virilization; 17α-OH = HTN + no sex hormones; 3β-HSD = salt-wasting + ambiguous in both sexes.
- CAIS vs. 5α-reductase: both 46,XY with female external phenotype at birth. CAIS → breasts, no hair, no virilization. 5α-reductase → virilization at puberty, no breasts.
- Klinefelter (47,XXY): tall, small firm testes, gynecomastia, infertility, ↑ FSH/LH, ↓ inhibin B.
- Turner (45,XO): short, webbed neck, primary amenorrhea, streak ovaries, bicuspid aortic valve, coarctation, horseshoe kidney.
- Always screen for associated renal anomalies (e.g., horseshoe kidney in Turner, ipsilateral renal agenesis with Müllerian anomalies).
🧠 Testing Your Understanding!
Testing Your Understanding — LO 5, Question 1
A 2-week-old 46,XX infant with ambiguous genitalia presents with vomiting, poor feeding, hyponatremia (122 mEq/L), hyperkalemia (6.8 mEq/L), and hypotension. Serum 17-hydroxyprogesterone is markedly elevated. Walk through the pathophysiology from enzyme block to salt-wasting crisis, and explain why the external genitalia are virilized.
Consider: adrenal zones, steroidogenesis pathways, precursor shunting, fetal adrenal androgen exposure timing
The 21-hydroxylase block prevents conversion of progesterone → 11-deoxycorticosterone (mineralocorticoid pathway) and 17-OH progesterone → 11-deoxycortisol (glucocorticoid pathway). Result: ↓ aldosterone (can't reabsorb Na, excrete K → hyponatremia, hyperkalemia, metabolic acidosis, hypotension) and ↓ cortisol (↓ negative feedback on ACTH → adrenal hyperplasia). The accumulated 17-OH progesterone is diverted into the androgen pathway (via 17,20-lyase → DHEA → androstenedione → testosterone → DHT) beginning early in gestation. First-trimester DHT exposure causes labioscrotal fusion and clitoromegaly — the 46,XX female is virilized. Management: hydrocortisone (replaces cortisol + suppresses ACTH), fludrocortisone (replaces mineralocorticoid), IV saline/glucose, and delayed feminizing genitoplasty. Consider: why would 46,XY infants appear normal at birth and present later with only salt-wasting? (External genitalia are already male by DHT; shunt-related androgens don't change phenotype.)
Testing Your Understanding — LO 5, Question 2
A 14-year-old girl presents with short stature, primary amenorrhea, and no breast development. Exam reveals a webbed neck, shield chest, and wide carrying angle. Labs show FSH 95 mIU/mL, LH 40 mIU/mL, estradiol <10 pg/mL. Echocardiogram shows a bicuspid aortic valve with mild coarctation; renal ultrasound reveals a horseshoe kidney. Name the diagnosis and explain the embryologic and cytogenetic mechanism linking all findings.
Consider: ovarian follicle atresia, SHOX haploinsufficiency, neural crest / cardiac development, kidney fusion embryology
In Turner syndrome, oocytes undergo accelerated atresia in utero, leaving streak gonads that cannot produce estrogen → failure of pubertal breast development and primary amenorrhea; gonadotropins (FSH > LH) are dramatically elevated because of absent estrogen feedback. SHOX gene haploinsufficiency on Xp is responsible for short stature and skeletal abnormalities (short metacarpals, cubitus valgus). Defective cervical lymphatic development causes intrauterine cystic hygroma, producing the webbed neck and pedal lymphedema. Abnormal neural-crest contribution to the cardiac outflow tract accounts for the bicuspid aortic valve (most common) and coarctation of the aorta. The horseshoe kidney reflects abnormal midline fusion of the metanephric blastemas, blocked during ascent by the IMA. Management: growth hormone for short stature; estrogen replacement to induce puberty and maintain bone density; cardiac surveillance; fertility via donor oocytes/ART. Consider: why is life-long cardiac imaging critical? (↑ Risk of aortic dissection, especially during pregnancy.)
Practice Questions
Lecture 02 — Urogenital System I & II · HoyaDocs Repro Block
LO 1 — Kidney Development (Q1–Q3)
Question 1
A fetal autopsy reveals small, non-functional kidneys with markedly hypoplastic collecting systems and a few dysmorphic nephrons. Genetic analysis identifies a homozygous loss-of-function mutation in GDNF. Which of the following developmental interactions failed first in this embryo?
A. Migration of primordial germ cells into the genital ridge
B. Signaling from the metanephric blastema to the mesonephric duct to induce the ureteric bud
C. Fusion of the urorectal septum with the cloacal membrane
D. Secretion of Müllerian Inhibiting Substance by Sertoli cells
E. Involution of transient fetal vessels during kidney ascent
GDNF (glial-derived neurotrophic factor) is made by the metanephric mesoderm and binds RET on the mesonephric duct to trigger ureteric bud budding — the first step in reciprocal induction. Without GDNF, no bud forms; without a bud, no collecting system branches and no inductive signals (WNT9B, FGF2) reach the blastema, so nephron differentiation is abortive. A (PGC migration) is distinct from renal development. C is the cloaca-split step. D is Sertoli/MIS signaling, unrelated to renal dev. E (vessel turnover) is an ascent problem, later. Tie-in: bilateral failure of this axis → renal agenesis → Potter sequence.
Question 2
A newborn is found to have a horseshoe kidney on prenatal ultrasound. The radiologist notes the kidney is located more caudally than expected. Which of the following embryological structures is most directly responsible for halting the ascent of this fused kidney?
A. Superior mesenteric artery
B. Celiac trunk
C. Inferior mesenteric artery
D. Renal arteries at L1
E. Common iliac arteries
Normally the metanephros ascends from the pelvis to L1–L2, rotating medially and picking up new arterial branches from the aorta as iliac vessels regress. In a horseshoe kidney, the fused lower-pole isthmus gets caught on the IMA at L3, so the kidney sits low in the abdomen. It is not the SMA or celiac trunk (those are more cranial and above the ascent endpoint). Clinical pearls: ↑ UTIs, stones, hydronephrosis, Wilms tumor, and classic association with Turner syndrome.
Question 3
During the 7th week of gestation, a developing kidney is examined. Cells derived from the metanephric blastema are labeled with a fluorescent tracer. Which of the following adult structures would be labeled?
A. Major and minor calyces
B. Renal pelvis
C. Collecting ducts
D. Proximal convoluted tubule and glomerulus
E. Ureter
Embryology split to memorize: metanephric blastema → nephron (Bowman's, glomerulus, PCT, LOH, DCT); ureteric bud → collecting system (collecting ducts, minor calyces, major calyces, renal pelvis, ureter). Choices A, B, C, and E are all ureteric-bud derivatives. A common Step 1 trap is confusing which structure makes what — particularly the collecting ducts, which feel like "the nephron's endpoint" but are actually ureteric-bud tissue.
LO 2 — Bladder & Duct Derivatives (Q4–Q6)
Question 4
A 48-year-old man presents with gross hematuria. Cystoscopy reveals a 2-cm tumor at the dome of the bladder in the midline. Histology shows mucin-producing glandular cells (adenocarcinoma). Which embryologic remnant most likely gave rise to this tumor?
A. Wolffian duct
B. Paramesonephric (Müllerian) duct
C. Urachus (allantois)
D. Cloacal membrane
E. Nephrogenic cord
The allantois connects the primitive bladder apex to the umbilicus; it normally obliterates to form the median umbilical ligament (urachus). Persistent segments can become cysts or sinuses and, rarely, undergo malignant transformation into adenocarcinoma at the bladder dome — distinct from the more common transitional cell carcinoma (urothelial) which arises from endodermal urothelium and usually occurs on the lateral walls or trigone. Wolffian duct (A) gives the trigone but not the dome. Müllerian (B) regresses in men. Cloacal membrane (D) is not associated with adenocarcinoma. The nephrogenic cord (E) makes kidneys.
Question 5
A 19-year-old phenotypic female is evaluated for primary amenorrhea. Karyotype is 46,XX. MRI shows absent uterus, absent cervix, absent upper 2/3 of the vagina, and a short blind-ending vagina. Ovaries and external genitalia are normal. Renal ultrasound reveals a unilateral absent kidney. The best explanation for the combination of genital and renal findings is:
A. Androgen receptor mutation (CAIS)
B. Failure of paramesonephric duct development (Müllerian agenesis)
C. 21-hydroxylase deficiency
D. 5α-reductase deficiency
E. Swyer syndrome (46,XY gonadal dysgenesis)
In MRKH, the paramesonephric ducts fail to form or canalize → no fallopian tubes, no uterus, no cervix, and no upper 2/3 of the vagina. The lower 1/3 (UG-sinus derived) is present but ends blindly. Ovaries (gonads, not ducts) are normal, so estrogen and pubertal breast development are normal. Because Müllerian development depends on the mesonephric duct as a guide, concurrent mesonephric defects produce ipsilateral renal anomalies (agenesis or ectopic kidney) in ~30–40% of MRKH patients. Karyotype 46,XX distinguishes MRKH from CAIS (46,XY; no uterus because MIS works normally). Testosterone is normal in MRKH.
Question 6
A 3-month-old boy is taken to surgery for bilateral cryptorchidism. In the pelvis, the surgeon unexpectedly identifies a small uterus and two fallopian tubes in addition to the two undescended testes. Karyotype is 46,XY; testosterone and DHT are normal. The most likely defective embryologic signal is:
A. Leydig cell testosterone secretion
B. 5α-reductase activity in the external genitalia
C. Sertoli cell secretion of Müllerian Inhibiting Substance
D. Androgen receptor signaling
E. GDNF-RET signaling during kidney induction
The patient has intact testes, normal testosterone (Leydig), and normal DHT (5α-reductase) — which explains the male external genitalia and internal Wolffian derivatives (vas deferens, epididymis). However, Müllerian-derived structures are present, indicating that either MIS (encoded by AMH) is deficient or its type II receptor is nonfunctional. Clinical tie-in: the retained Müllerian structures often tether the testes, causing cryptorchidism. Contrast with CAIS (D; androgen receptor broken — no Wolffian or Müllerian) and 5α-reductase (B; female-appearing external genitalia at birth).
LO 3 — Genital Differentiation (Q7–Q9)
Question 7
A researcher selectively knocks out expression of the androgen receptor in a developing 46,XY embryo while leaving all other hormonal pathways intact. Which of the following best describes the predicted external and internal phenotype at birth?
A. Male internal ducts, male external genitalia
B. Female internal ducts, female external genitalia
C. Neither Wolffian nor Müllerian ducts; female external genitalia
D. Male internal ducts, female external genitalia
E. Female internal ducts, ambiguous external genitalia
The androgen receptor is the single relay for both testosterone (acts on Wolffian duct) and DHT (acts on external genitalia + prostate). Knocking out AR silences both T and DHT effects. Sertoli cells still make MIS, which causes Müllerian regression — so there's no uterus or upper vagina either. External genitalia default to female because there is no functional DHT signaling. Clinical tie-in: CAIS patients are phenotypic females with bilateral testes (often inguinal/abdominal) and a short blind vagina; breast development occurs at puberty because aromatase converts testosterone to estradiol.
Question 8
A term infant is born with ambiguous genitalia, including an enlarged phallic structure and partially fused labioscrotal folds. Karyotype returns 46,XX. Which of the following hormones, secreted by the fetus itself, is most likely responsible for the virilized external phenotype?
A. Anti-Müllerian hormone (MIS)
B. Estradiol
C. Dihydrotestosterone (DHT)
D. Aldosterone
E. Cortisol
In 46,XX first-trimester androgen excess (most commonly 21-hydroxylase CAH, but also 11β-hydroxylase CAH or exogenous sources), the fetal adrenal makes large amounts of androstenedione / testosterone, which is converted to DHT locally in the genital skin. DHT drives labioscrotal fusion, clitoromegaly, and a common urogenital sinus. MIS (A) regresses Müllerian ducts but doesn't virilize. Estradiol (B) doesn't cause virilization. Aldosterone (D) / cortisol (E) aren't androgens. Timing matters — first-trimester exposure produces fusion; third-trimester exposure causes clitoromegaly alone.
Question 9
A 15-year-old with a 46,XY karyotype presents with primary amenorrhea, absent breast development, and Tanner I pubic hair. Labs reveal FSH 70 mIU/mL, LH 45 mIU/mL, testosterone < 20 ng/dL, estradiol < 10 pg/mL. MRI shows a small uterus and bilateral streak gonads. Which of the following best explains this presentation?
A. Klinefelter syndrome (47,XXY)
B. Complete Androgen Insensitivity Syndrome
C. 5α-reductase deficiency
D. Swyer syndrome (46,XY gonadal dysgenesis)
E. Müllerian agenesis (MRKH)
Without SRY, the bipotential gonad cannot become a testis → streak gonads form instead. No Sertoli cells → no MIS → Müllerian ducts persist (small uterus, tubes, upper vagina). No Leydig cells → no testosterone → Wolffian ducts regress and external genitalia default to female. Because there is no negative feedback from gonadal steroids, gonadotropins are elevated (hypergonadotropic hypogonadism). Contrast with CAIS (B): 46,XY, no uterus (MIS works), high-normal testosterone, breasts develop via aromatase. Swyer patients need estrogen replacement for puberty and gonadectomy (dysgenic gonads → ↑ risk of gonadoblastoma).
LO 4 — Urinary & Genital Anomalies (Q10–Q13)
Question 10
A neonate is born at 36 weeks gestation with flattened facies, clubbed feet, low-set ears, and respiratory failure in the delivery room. Ultrasound done during pregnancy showed severe oligohydramnios and the fetal bladder could not be visualized. Autopsy is requested. Which of the following is the most likely direct cause of death?
A. Renal failure
B. Pulmonary hypoplasia
C. Congenital heart disease
D. Hyperkalemia from acute tubular necrosis
E. Sepsis from urinary tract infection
After the second trimester, amniotic fluid comes primarily from fetal urine. Bilateral renal agenesis or severe obstructive uropathy prevents this → oligohydramnios → mechanical compression of the fetus (flat facies, clubbed feet, limb deformities) and — critically — failure of lung distension. The lungs require amniotic fluid movement and fetal breathing to develop normal alveolar architecture. Pulmonary hypoplasia causes fatal respiratory failure at birth — the placenta handled waste during gestation, so renal failure (A) is not the killer. This is the classic Potter sequence.
Question 11
A 6-year-old girl presents with continuous urinary dampness requiring pads despite normal voiding every 2–3 hours. Exam is unremarkable except for mild perineal dermatitis. Renal ultrasound reveals a duplex collecting system on the left kidney. Which additional finding is most likely explaining her continuous incontinence?
A. The upper-pole ureter inserts into the urethra distal to the external sphincter
B. The lower-pole ureter is refluxing urine back into the kidney
C. The upper-pole ureter inserts at the trigone with vesicoureteral reflux
D. Bilateral renal agenesis with oligohydramnios
E. A patent urachus connecting bladder to umbilicus
By the Weigert-Meyer rule, in a duplex kidney the upper-pole ureter inserts inferiorly and medially (often ectopically) while the lower-pole ureter inserts normally at the trigone. In females, an ectopic ureter can open below the external urethral sphincter (urethra distal to sphincter, vestibule, or vagina), bypassing continence — producing continuous dribbling despite an otherwise normal voiding pattern from the intact ureter. VUR (B/C) causes recurrent UTIs but not continuous incontinence. Bilateral renal agenesis (D) would not be compatible with a 6-year-old. Patent urachus (E) would drain from the umbilicus.
Question 12
An 18-month-old boy undergoes surgery for a right-sided undescended testis found in the inguinal canal. Which of the following is true regarding his long-term risk and the purpose of early intervention?
A. Testosterone production from the affected testis is severely impaired
B. Early orchidopexy substantially reduces future risk of seminoma
C. Delaying surgery until age 10 has no impact on fertility
D. The contralateral descended testis is at increased risk of torsion
E. Elevated testosterone levels explain the cryptorchidism
An undescended testis sits in the warmer abdominal/inguinal environment, which damages heat-sensitive seminiferous tubules — reflected in ↓ inhibin B and ↑ FSH. Leydig cells are heat-resistant, so testosterone is usually normal (A incorrect). The longer the testis remains undescended, the greater the risk of seminoma (most common germ cell tumor in abdominal testis). Orchidopexy before age 1 (ideally) maximizes preservation of fertility and reduces malignancy risk, though does not normalize it completely. The ipsilateral testis — not the contralateral — is at ↑ torsion risk (D incorrect). Cryptorchidism is associated with low, not elevated, androgen axis activity (E incorrect).
Question 13
A newborn male is examined and the urethral meatus is found on the ventral surface of the penile shaft midway between the glans and the scrotum. Which of the following is the most appropriate next step in management?
A. Immediate circumcision to reduce UTI risk
B. Defer circumcision; refer to pediatric urology for planned urethroplasty
C. Order karyotype to assess for disorders of sexual differentiation
D. Begin testosterone therapy to promote urethral fold fusion
E. Obtain a brain MRI to rule out Kallmann syndrome
The finding is penile hypospadias — failure of the urogenital (urethral) folds to fuse ventrally, driven by insufficient DHT effect during fetal development. Definitive repair (urethroplasty) is typically performed between 6 and 18 months and often requires the foreskin as a tissue graft. Early circumcision destroys this reconstruction option. Routine karyotype (C) isn't indicated for isolated mild hypospadias, but is warranted if cryptorchidism, ambiguous genitalia, or bilateral non-palpable testes are present (to rule out virilized 46,XX CAH). Testosterone therapy (D) is not standard and would not reverse the structural defect. Brain MRI (E) is not indicated — Kallmann involves hypothalamic GnRH deficiency and anosmia.
LO 5 — Disorders of Sex Development (Q14–Q18)
Question 14
A 10-day-old infant is brought to the ED with vomiting, lethargy, and failure to thrive. Exam reveals an enlarged clitoris with partially fused labioscrotal folds. Labs: Na 121 mEq/L, K 6.7 mEq/L, glucose 42 mg/dL, 17-hydroxyprogesterone markedly elevated. Which of the following medications would most directly address both the cortisol deficiency and the hormonal shunting toward androgens?
A. Fludrocortisone
B. Hydrocortisone
C. Spironolactone
D. Dexamethasone suppression test dose
E. Testosterone
In 21-hydroxylase deficiency (classic salt-wasting CAH), cortisol production is impaired. Loss of negative feedback causes ↑ ACTH, which drives adrenal hyperplasia and push more precursors through the only pathway still open — androgens. Hydrocortisone replaces cortisol and suppresses ACTH, reducing shunting. Fludrocortisone (A) is also needed to replace aldosterone and correct salt-wasting, but it does not address cortisol or androgen shunting. Spironolactone (C) blocks aldosterone — opposite of what we want. Dexamethasone suppression (D) is diagnostic, not therapeutic. Testosterone (E) is dangerous — the patient already has androgen excess.
Question 15
A 16-year-old 46,XY is brought to the clinic by her parents because she has not started menstruating. She is tall for age, has Tanner IV breast development, sparse pubic hair, and no uterus on pelvic imaging. Bilateral inguinal masses are palpable. Which of the following is the most appropriate long-term management plan?
A. Immediate gonadectomy and testosterone replacement
B. Gonadectomy after completion of puberty; raise as female; estrogen replacement
C. Orchidopexy and raise as male
D. Hydrocortisone and fludrocortisone therapy
E. Growth hormone and fludrocortisone
The clinical pattern (46,XY, female phenotype with breast development, sparse pubic hair, absent uterus, and inguinal testes) is classic CAIS. Testes allow pubertal feminization via aromatase converting testosterone to estradiol → breast development. Because retained undescended testes have ↑ malignancy risk (germ-cell tumor / gonadoblastoma), they are removed after puberty completes natural feminization. Estrogen replacement is then lifelong. Pre-pubertal gonadectomy (A) loses pubertal feminization benefit. Orchidopexy (C) and masculinization is not appropriate — the androgen receptor cannot respond to testosterone. CAH treatments (D, E) are not the issue here.
Question 16
A newborn 46,XY has ambiguous external genitalia with a small phallus and penoscrotal hypospadias; internal imaging shows normal epididymis, vas deferens, and seminal vesicles. At age 12, dramatic virilization occurs — voice deepens, muscle mass increases, and the phallus enlarges. Labs show testosterone 650 ng/dL, DHT 15 ng/dL (ref 30–85). Which enzyme is deficient?
A. 21-hydroxylase
B. 11β-hydroxylase
C. 17α-hydroxylase
D. 5α-reductase
E. Aromatase
The Wolffian-derived internal ducts (SEED) are normal because they depend on testosterone directly — and testosterone is normal. The external genitalia require DHT, which is deficient → ambiguous/female-appearing at birth with penoscrotal hypospadias. At puberty, the massive testosterone surge partially drives virilization through residual T action and alternative DHT-generating pathways ("female at birth, male at puberty"). Hallmark lab: ↑ testosterone-to-DHT ratio. Not CAH (A/B/C — those have abnormal adrenal steroids and ambiguous findings at birth in different patterns). Aromatase deficiency (E) would cause maternal + fetal virilization and no estrogens at puberty (different presentation).
Question 17
A 24-year-old man presents with infertility. Exam reveals tall stature, long extremities, and small, firm testes bilaterally. He also notes mild gynecomastia. Labs: FSH 42 mIU/mL (high), LH 18 mIU/mL (high), testosterone 250 ng/dL (low-normal), estradiol 55 pg/mL (high), inhibin B low. Karyotype is pending. What is the most likely karyotype?
A. 45,XO
B. 46,XY with SRY deletion
C. 46,XX with SRY translocation
D. 47,XXY
E. 47,XYY
Classic Klinefelter triad: tall stature, small firm testes, and infertility. Seminiferous tubule dysgenesis → ↓ inhibin B → ↑ FSH; Leydig cell impairment → low-normal testosterone and ↑ LH. Elevated estradiol from aromatization of residual testosterone (and decreased SHBG) causes gynecomastia. 45,XO (A) is Turner — short female phenotype with primary amenorrhea. SRY deletion (B) causes Swyer syndrome. SRY translocation (C) = 46,XX male — small testes but short stature. 47,XYY (E) is associated with tall stature and mild behavioral features, but testicular size and fertility are usually normal. Klinefelter is the most common cause of male infertility from a chromosomal etiology.
Question 18
A 12-year-old girl presents with short stature and primary amenorrhea. Exam reveals a webbed neck, shield chest with widely spaced nipples, and a wide carrying angle. Echocardiogram shows a bicuspid aortic valve; renal ultrasound shows a horseshoe kidney. Which of the following hormone patterns is most consistent with this diagnosis?
A. ↑ FSH, ↑ LH, ↓ estradiol
B. ↓ FSH, ↓ LH, ↓ estradiol (hypothalamic)
C. ↑ FSH, ↑ LH, ↑ estradiol
D. ↑ 17-OH progesterone, ↓ Na, ↑ K
E. Normal FSH/LH, high prolactin
Turner syndrome (45,XO) features accelerated ovarian follicle atresia → streak gonads → no estrogen synthesis → primary amenorrhea and failure of secondary sex characteristics. Loss of estrogen negative feedback on the pituitary causes hypergonadotropic hypogonadism (↑↑ FSH, ↑ LH, ↓ estradiol). Classic associations: bicuspid aortic valve (most common cardiac anomaly), coarctation, and horseshoe kidney — all shown here. Management: growth hormone for height, estrogen replacement at puberty, and close cardiac follow-up (↑ aortic dissection risk). B is hypothalamic hypogonadism (e.g., Kallmann) — but Turner is primary ovarian failure. D is 21-OH CAH. E suggests prolactinoma.