Müllerian ducts use mesonephric ducts as a physical scaffold; the ureteric bud sprouts from the caudal mesonephric duct. A unilateral Wolffian/paramesonephric disruption can cause BOTH bicornuate uterus AND ipsilateral renal agenesis
Persistent Müllerian Duct Syndrome (PMDS) — defective MIS (AMH mutation) or MIS type II receptor; T and external phenotype are normal
FEMALE — without hormones (T, MIS), Müllerian ducts persist and external genitalia default to unfused/female; male differentiation requires ACTIVE hormonal input
LO 3
ON EXAM — SRY on Yp → TDF, Sertoli → MIS; Leydig → T → 5α-reductase → DHT; internal (T) vs external (DHT); bipotential genitalia → penis vs clitoris, scrotum vs labia majora; CAIS vs 5α-reductase; Swyer vs 46,XX male
Gonad formation (LO3)
Endoderm of the yolk-sac wall; migrate along dorsal mesentery of the hindgut → seed the genital ridge at week 5
Coelomic epithelium invades the mesodermal core of the genital ridge
Week 6 — paramesonephric (Müllerian) ducts also forming alongside the mesonephric ducts
UG sinus, genital tubercle, urethral folds, labioscrotal swellings, prostate
Virilization of EXTERNAL male genitalia + prostate
Aromatase
Gonads, placenta, adipose, skin
Testosterone → estradiol
Estradiol drives female secondary sex characteristics at puberty (relevant when testes are retained, as in CAIS)
EXAM TRAP: Testosterone acts on internal Wolffian ducts. DHT acts on external genitalia + prostate. Both use the same androgen receptor — AIS blocks BOTH.
Bipotential external genitalia — fill derivative with / without DHT (LO3)
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
Gubernaculum
Gubernaculum testis (guides testicular descent through inguinal canal)
Labioscrotal fusion + clitoromegaly + common urogenital sinus → full genital virilization (e.g., CAH)
Only clitoromegaly (no fusion) — the labioscrotal folds have already 'decided' their fate by then
Testes present; no T → Wolffian regresses (no SEED); MIS works → no Müllerian → NEITHER internal duct. External = female (no DHT)
Testes present; T works → Wolffian SEED present; MIS works → no Müllerian; external = female/ambiguous at birth, virilizes at puberty ('female at birth, male at puberty')
LO 4
ON EXAM — Potter sequence (cause of death = pulmonary hypoplasia), horseshoe kidney caught at IMA, pelvic kidney (adrenal stays put), duplex kidney + Weigert-Meyer, ectopic ureter + female incontinence, cryptorchidism (heat ↓ inhibin B), hypospadias (ventral, do NOT circumcise) vs epispadias (dorsal, bladder exstrophy)
Potter sequence (LO4)
(1) Bilateral Renal Agenesis (BRA); (2) ARPKD; (3) Bilateral obstructive uropathy (e.g., posterior urethral valves); (4) Esophageal atresia or other cause preventing fetus from swallowing amniotic fluid (VACTERL)
P — Pulmonary hypoplasia | O — Oligohydramnios | T — Twisted face | T — Twisted skin/extremities | E — Extremity defects | R — Renal failure (in utero)
Structural kidney anomalies (LO4)
Anomaly
Mechanism
Classic finding / clinical
Multicystic dysplastic kidney (MCDK)
Failed reciprocal induction → renal parenchyma replaced by non-functioning cysts of various sizes
'Bunch of grapes' on prenatal US; most common cause of abdominal mass in a newborn; usually unilateral (bilateral = Potter)
Horseshoe kidney
Midline fusion of the lower poles of metanephric blastemas, caught on the inferior mesenteric artery (IMA) during ascent
Premature bifurcation of the ureteric bud → two ureters drain a single kidney (most common renal structural anomaly)
Upper-pole ureter inserts INFERIOR + MEDIAL (often ectopic, often with ureterocele); Lower-pole ureter inserts normally at the trigone (often with VUR)
Distal ureter opens BELOW the external urethral sphincter (urethra distal to sphincter, vagina, or vestibule) → bypasses voluntary continence → constant dribbling despite normal voiding from the other ureter
Ureter opens ABOVE the external sphincter (into vas deferens, seminal vesicle, epididymis) — so no continuous leakage; risk of epididymitis or fertility issues
'Cobra-head sign' on imaging (balloon-like cystic dilation of distal intramural ureter); often associated with duplex kidney
Uretero-Pelvic Junction (UPJ) obstruction
Testicular descent & cryptorchidism (LO4)
(1) Gubernaculum testis (caudal ligament guides direction) + (2) Processus vaginalis (peritoneal outpouching that creates the path through the inguinal canal)
External oblique aponeurosis → external spermatic fascia | Internal oblique → cremaster muscle + cremasteric fascia | Transversalis fascia → internal spermatic fascia
Normal: obliterates after descent (remnant = tunica vaginalis around testis). Patent → indirect inguinal hernia OR communicating hydrocele
Hydrocele TRANSILLUMINATES; indirect inguinal hernia does NOT
T = NORMAL (Leydig cells are heat-resistant); inhibin B = LOW (Sertoli heat damage); FSH = ELEVATED (loss of inhibin B feedback) — classic cryptorchidism lab pattern
↑ risk of germ cell tumors — especially SEMINOMA in an abdominal testis; early orchidopexy reduces this risk
Before 1 year of age (after 6-month observation window for spontaneous descent)
Hypospadias vs epispadias (LO4)
Mnemonic: Hypo = below (ventral). Epi = above (dorsal). One key management pearl: do NOT circumcise a hypospadias.
Feature
Hypospadias
Epispadias
Mechanism
Failure of urogenital (urethral) folds to fuse ventrally along the penile shaft
X-linked mutation of androgen receptor (AR) — T & DHT can't signal
Testes (often inguinal or intra-abdominal)
NEITHER — no Müllerian (MIS works) + no Wolffian (AR broken); short blind vagina
Female
Breasts develop (aromatase → estradiol); sparse pubic/axillary hair; NO virilization
5α-reductase deficiency
Autosomal recessive — cannot convert T → DHT
Testes
Wolffian present (T-dependent); no Müllerian (MIS works)
Female / ambiguous at birth (penoscrotal hypospadias, small phallus, blind vagina)
VIRILIZATION (deep voice, phallic growth, muscle mass); NO breast development
Swyer (46,XY gonadal dysgenesis)
SRY deletion or mutation → no TDF → no testes form
Streak gonads (non-functional)
Müllerian (no MIS, no T)
Female
NO virilization, NO breast development (needs estrogen replacement); high gonadoblastoma risk → gonadectomy
Testes still make T → aromatase converts to ESTRADIOL → breast development. But the androgen receptor is broken → no pubic/axillary hair growth (which requires AR signaling)
Elevated testosterone-to-DHT ratio (T:DHT ↑)
AFTER puberty — testes allow feminization via aromatase-derived estradiol. Gonadectomy performed post-pubertally to reduce testicular malignancy risk
Karyotype: CAIS = 46,XY (testes in inguinal canal, ↑T); MRKH = 46,XX (normal ovaries, normal T/estradiol). Both present with primary amenorrhea + blind vagina + absent uterus
Chromosomal DSDs (LO5)
47,XXY — Tall (eunuchoid proportions, long extremities); small FIRM testes (hyalinized seminiferous tubules); gynecomastia; developmental delay / learning difficulty
T = ↓ or low-normal | FSH = ↑↑ (loss of inhibin B feedback), LH = ↑ (partial Leydig failure) | inhibin B = ↓ (Sertoli dysfunction) | Estradiol = ↑ (↑ aromatase activity → gynecomastia, female fat distribution)
INFERTILE (azoospermia from hyalinized tubules). Cancer risk: ↑ breast cancer vs other men; ↑ extragonadal (mediastinal) germ cell tumors
45,XO — Short stature (SHOX haploinsufficiency); webbed neck (cystic hygroma); shield chest with widely-spaced nipples; cubitus valgus; lymphedema of hands/feet in infancy; low posterior hairline
SHOX gene (Short stature HOmeoboX-containing, on Xp) — haploinsufficiency drives short stature and skeletal features
LO 1–5 · USMLE Step 1 Style
8 questions — cover all 5 LOs. Use these after finishing all tabs.
Q1. A research team generates a mouse embryo in which the ureteric bud is surgically ablated on the right side at the equivalent of human gestational week 5, while the left side is left intact. At birth, imaging on the right side would MOST likely show which of the following?
A. A normal-sized right kidney with a duplicated ureter
B. A dysplastic right kidney with cystic remnants and no ureter
C. Complete absence of the right kidney and right ureter
D. A pelvic right kidney with variant iliac blood supply
E. A horseshoe-shaped kidney with bilateral ureters
C — Complete absence of the right kidney and right ureter (unilateral renal agenesis). Without a ureteric bud, there is no WNT9B/FGF2 signal to induce the metanephric blastema, so no nephrons condense. Reciprocal induction fails on that side → no collecting system AND no functional nephrons. The contralateral side compensates (hypertrophy); unilateral disease is usually compatible with life. Bilateral bud failure → bilateral renal agenesis → Potter sequence (oligohydramnios → pulmonary hypoplasia).
Q2. A neonate dies shortly after birth from respiratory failure. Autopsy reveals flattened facies, clubbed feet, and severely hypoplastic lungs. Prenatal ultrasound had shown oligohydramnios and bilateral absent kidneys. Which of the following BEST explains the cause of death?
A. Acute renal failure from bilateral renal agenesis
B. Uremia-induced encephalopathy
C. Pulmonary hypoplasia due to absent fetal urine → oligohydramnios → failed lung distension
D. Pulmonary surfactant deficiency from premature birth
E. Cardiac defects from failed neural crest migration
C — Pulmonary hypoplasia from oligohydramnios (Potter sequence). The placenta filters fetal blood during gestation, so the fetus does not die of uremia. After ~20 weeks, amniotic fluid comes almost entirely from fetal urine; without kidneys, oligohydramnios develops → mechanical compression (Potter facies, limb deformities) AND failed amniotic-fluid-driven lung distension → pulmonary hypoplasia = actual cause of neonatal death.
Q3. A 16-year-old girl presents with primary amenorrhea. Pelvic imaging reveals a bicornuate uterus, and renal ultrasound shows an absent right kidney. Which single embryologic principle BEST explains the co-occurrence of these two findings?
A. Failure of neural crest migration
B. Failed urorectal septum fusion
C. Abnormal SRY translocation
D. The mesonephric duct scaffolds paramesonephric descent and sprouts the ureteric bud — one disruption, two findings
E. Failed sinovaginal node fusion
D — The mesonephric (Wolffian) duct is the shared scaffold. The paramesonephric (Müllerian) ducts descend alongside the mesonephric ducts; partial fusion failure produces a bicornuate uterus. The ureteric bud sprouts from the caudal mesonephric duct; its failure → unilateral renal agenesis. A single Wolffian/paramesonephric disruption can produce both defects on the same side. Always order a renal ultrasound when a Müllerian anomaly is identified.
Q4. A phenotypically male newborn is undergoing surgery for bilateral non-palpable testes. The surgeon unexpectedly finds a uterus and two fallopian tubes. Karyotype is 46,XY; testosterone, DHT, and LH are all normal for age. Which embryologic event failed?
A. Leydig cells failed to produce testosterone
B. Neural crest migration into the distal cloaca failed
C. Sertoli cells failed to produce MIS, or target tissues lack the MIS receptor
D. The metanephric blastema failed to receive WNT9B
E. 5α-reductase activity was absent in target tissues
C — Persistent Müllerian Duct Syndrome (PMDS). Sertoli cells normally produce MIS (AMH), which triggers regression of the paramesonephric ducts at week 7–8. If MIS is absent (AMH gene mutation) or the MIS type II receptor is nonfunctional, Müllerian structures persist despite otherwise normal male development (testosterone from Leydig → Wolffian SEED; DHT → external male genitalia). Retained Müllerian structures often tether the testes intra-abdominally → cryptorchidism.
Q5. A 14-year-old phenotypically female patient presents with primary amenorrhea. Exam reveals Tanner IV breasts, sparse pubic hair, and a short blind-ending vagina. Imaging shows no uterus or fallopian tubes; bilateral inguinal masses are present. Karyotype is 46,XY. Which ONE of the following best explains both the absent uterus AND the breast development?
A. Functional Sertoli cells (MIS active) AND functional aromatase converting testosterone to estradiol
B. Failed SRY signaling with streak gonads producing endogenous estrogen
C. Defective 5α-reductase with normal aromatase
D. 21-hydroxylase deficiency with virilization
E. Functional Müllerian ducts with exogenous androgen exposure
A — Complete Androgen Insensitivity (CAIS). Testes form normally (SRY present). Sertoli cells make MIS → Müllerian regression → no uterus, no upper vagina. Leydig cells produce testosterone normally or high, but the mutant androgen receptor cannot respond → Wolffian ducts regress and external genitalia default to female. Peripheral aromatase converts the high circulating testosterone to estradiol → breast development at puberty. Sparse pubic/axillary hair reflects lost androgen receptor signaling. Inguinal masses = undescended testes.
Q6. A 46,XY infant is born with ambiguous, predominantly female-appearing external genitalia: a small phallus, penoscrotal hypospadias, labial fusion, and a blind-ending vagina. Testosterone level is high-normal and LH is mildly elevated. By late adolescence, the patient has developed increased muscle mass, a deepened voice, and growth of the phallus, but no breast development. What lab finding would confirm the diagnosis?
A. Elevated 17-hydroxyprogesterone
B. Elevated testosterone-to-DHT ratio
C. Karyotype showing 47,XXY
D. Elevated FSH with low inhibin B
E. Absent response to exogenous HCG stimulation
B — Elevated testosterone-to-DHT ratio (5α-reductase deficiency). Autosomal recessive loss of 5α-reductase prevents T → DHT conversion in target tissues. Internal Wolffian ducts (T-dependent) form normally; external genitalia (DHT-dependent) do not masculinize until the pubertal T surge directly overwhelms the block. Classic "female at birth, male at puberty"; androgen receptor works (hence muscle/phallus growth, no breast development).
Q7. A 2-week-old 46,XX infant with ambiguous genitalia presents with vomiting, poor feeding, hyponatremia, hyperkalemia, and hypotension. Serum 17-hydroxyprogesterone is markedly elevated. Which of the following is the MOST appropriate immediate management?
A. Testosterone replacement therapy to masculinize genitalia
B. Hydrocortisone + fludrocortisone + IV fluids/glucose
C. Surgical gonadectomy
D. Methylprednisolone taper only
E. Spironolactone and potassium restriction
B — Hydrocortisone + fludrocortisone + IV fluids. This is classic salt-wasting 21-hydroxylase deficiency. Hydrocortisone replaces cortisol AND suppresses ACTH (reducing androgen shunting). Fludrocortisone replaces aldosterone to correct the salt-wasting (hyponatremia, hyperkalemia). Giving testosterone (A) to a virilized 46,XX would worsen virilization. Gonadectomy (C) has no role — gonads are ovaries. Spironolactone (E) would further deplete aldosterone activity.
Q8. A 15-year-old girl is evaluated for short stature, primary amenorrhea, webbed neck, and shield chest. Echocardiography shows a bicuspid aortic valve with mild coarctation. Renal ultrasound reveals a horseshoe kidney. Which embryologic mechanism BEST unifies the renal finding with the rest of her presentation?
A. Failure of neural crest migration into the distal cloaca
B. Fusion of the two metanephric blastemas across the midline, trapped under the IMA during ascent
C. Failed reciprocal induction between the ureteric bud and metanephric blastema
D. Failure of the urorectal septum to fuse with the cloacal membrane
E. Abnormal rotation of the kidneys with failure of the nephrogenic cord
B — Midline fusion of the metanephric blastemas, caught under the inferior mesenteric artery during ascent. The patient has Turner syndrome (45,XO): short stature (SHOX haploinsufficiency), streak gonads with hypergonadotropic hypogonadism, webbed neck (cystic hygroma), bicuspid aortic valve + coarctation, and horseshoe kidney. The horseshoe is formed when the lower poles fuse across the midline; the isthmus catches on the IMA → the kidney sits low at L3–L5. Turner patients should have lifelong cardiac monitoring for aortic dissection and renal monitoring for reflux/UTIs.