5 vignettes · Gastrulation, Sox17/Foxa2/Nodal, gut tube landmarks
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Instructions: Read each vignette carefully, select the single best answer (A–E). Target: 90 seconds per question. Answers and explanations lock after each click; scores appear in the Score Report tab.
Question 1 · Origin of Definitive Endoderm
A developmental biologist fate-maps cells in a week-3 human embryo. Cells that ingress through the primitive streak and displace the underlying hypoblast are traced forward in time. To which of the following adult structures will these cells MOST likely contribute?
ADermis of the skin
BSkeletal muscle of the limbs
CEpithelial lining of the small intestine
DYolk sac extraembryonic membrane
ECentral nervous system neurons
Answer: C. Epiblast cells that ingress through the primitive streak and displace the hypoblast become definitive endoderm, the source of the entire gut epithelium (esophagus to upper anal canal) plus LLGP (liver, lung, gallbladder, pancreas). The hypoblast itself contributes only to extraembryonic yolk sac — it does NOT form gut. Dermis/limb muscle = mesoderm; CNS = ectoderm.
Question 2 · Master Transcription Factor
A transgenic mouse embryo carries a conditional knockout of a transcription factor. At embryonic day 8.5 the embryo shows complete absence of a recognizable gut tube; the ectoderm and mesoderm are otherwise intact. Which transcription factor was MOST likely ablated?
APax6
BMyoD
CSox17
DBrachyury (T)
ESonic hedgehog (Shh)
Answer: C.Sox17 is the master TF for definitive endoderm specification. Without Sox17, ingressing epiblast cells fail to commit to endoderm — no gut tube forms. Foxa2 is a pioneer TF acting alongside Sox17. Brachyury specifies mesoderm; Pax6 is eye/CNS; MyoD is muscle; Shh is patterning (polarity) but not endoderm specification per se.
Question 3 · Hypoblast Is Not Gut
A medical student is asked to identify the correct embryologic origin of the epithelium lining the jejunum. Which of the following statements is TRUE?
AThe hypoblast displaces epiblast cells to form the gut tube
BThe jejunal epithelium derives from splanchnic mesoderm
CEpiblast-derived cells migrate through the primitive streak, displace the hypoblast, and become definitive endoderm
DNeural crest cells give rise to gut epithelium in the midgut
EThe yolk sac endoderm becomes the jejunal lining in week 4
Answer: C. Correct sequence: bilaminar disc (epiblast + hypoblast) → primitive streak appears → epiblast cells ingress → Sox17⁺ cells displace hypoblast → become definitive endoderm (gut). Hypoblast = extraembryonic yolk sac lining only. Gut wall (smooth muscle, CT) comes from splanchnic mesoderm; epithelium is endoderm. NCC gives rise to enteric ganglia, not epithelium.
Question 4 · Buccopharyngeal / Cloacal Membranes
A newborn has imperforate anus discovered on routine examination. Which embryologic event most likely failed?
AFailure of the buccopharyngeal membrane to rupture in week 4
BFailure of the cloacal membrane to rupture in week 7
CFailed recanalization of the duodenum
DIncomplete lateral folding of the embryo
EFailed migration of the vagal neural crest
Answer: B. Both ends of the primitive gut are closed by bilaminar (ecto + endo, no mesoderm) membranes. The buccopharyngeal membrane caps the cranial end and ruptures in week 4 to open the mouth. The cloacal membrane caps the caudal end and ruptures in week 7 to open the anus and urogenital openings. Failure → imperforate anus (part of VACTERL).
Question 5 · Gut Tube from Body Folding
During embryonic folding, the flat trilaminar disc is transformed into a tube-within-a-tube body plan. Which combination of folding movements is required to convert the open yolk sac into a closed primitive midgut?
AOnly cranial-caudal folding
BOnly lateral folding
CBoth cranial-caudal and lateral folding, with persistence of the vitelline duct
DRotation of the embryo 90° clockwise around its long axis
EInfolding of neural crest cells into the gut endoderm
Answer: C. The flat trilaminar disc becomes a cylinder via cranial-caudal folding (creates foregut + hindgut pockets) PLUS lateral folding (closes body wall, pinches the yolk sac). The central portion becomes the midgut, still connected to the yolk sac by the vitelline duct (future Meckel's if it persists). Splanchnic mesoderm wraps the gut tube; somatic mesoderm forms the body wall.
Block 2 — Gut Regions · Arterial Supply · Mesenteries
Block 2 focus: Arterial supply by region, foregut accessories, watershed ischemia, pectinate line anatomy, ventral/dorsal mesenteries.
Question 6 · Foregut/Midgut Boundary
A 45-year-old woman undergoes ERCP for obstructive jaundice. Endoscopy identifies the major duodenal papilla (ampulla of Vater) within the descending duodenum. The mucosa proximal to this papilla is supplied by which artery, and from which embryologic region does it derive?
ACeliac trunk · foregut
BSuperior mesenteric artery · midgut
CInferior mesenteric artery · hindgut
DGastroduodenal artery only · midgut
EInternal pudendal · hindgut
Answer: A. The foregut/midgut boundary is the ampulla of Vater (major duodenal papilla) — NOT the pylorus. Proximal to the ampulla → foregut → celiac trunk. Distal to the ampulla → midgut → SMA. The 2nd part of the duodenum contains the ampulla; D1 + proximal D2 are foregut, distal D2 + D3 + D4 are midgut.
Question 7 · Watershed Ischemia After AAA
A 73-year-old man develops LLQ pain and bloody diarrhea 36 hours after elective open AAA repair, during which the inferior mesenteric artery was ligated. Colonoscopy demonstrates segmental mucosal ischemia; the most vulnerable colonic segment is located at the:
ACecum (ileocecal area)
BHepatic flexure
CSplenic flexure (SMA/IMA watershed)
DMid-transverse colon
EAnal canal below pectinate line
Answer: C. The splenic flexure is the SMA/IMA watershed — the distal-most territory of both arteries and lowest perfusion pressure. When IMA is sacrificed during AAA repair, the splenic flexure is first to ischemize. Classic buzzwords: "thumbprinting" on plain film, bloody diarrhea, LLQ pain. The rectum below pectinate gets blood from internal iliac (not aortic branches), so it's usually spared.
Question 8 · Pectinate Line + Hemorrhoid Type
A 58-year-old man presents with painless, bright red blood per rectum after straining. Anoscopy identifies a soft mass above the dentate line. Which set of features corresponds to this finding?
ADerived from endoderm · superior rectal vein (portal drainage) · visceral (painless) innervation · internal iliac nodes
A surgeon resects the liver. During the procedure he clamps the free edge of the lesser omentum (the hepatoduodenal ligament) — the "Pringle maneuver." The lesser omentum is a derivative of the ventral mesentery. Which of the following correctly characterizes the ventral mesentery?
AExtends along the entire length of the gut tube
BExists only in the foregut, and its major derivatives are the falciform ligament and lesser omentum
CExists only in the midgut and gives rise to the mesentery proper
DExists only in the hindgut and gives rise to the sigmoid mesocolon
EGives rise to the gastrosplenic ligament and greater omentum
Answer: B.Ventral mesentery = foregut ONLY (esophagus to proximal duodenum). Its derivatives are the falciform ligament (liver → anterior body wall) and the lesser omentum (hepatoduodenal + hepatogastric). The hepatoduodenal ligament's free edge contains the portal triad (PV, HA, CBD) — site of the Pringle maneuver. Greater omentum and gastrosplenic ligament come from dorsal mesentery.
Question 10 · SAD PUCKER / Retroperitoneal
A 52-year-old alcoholic with severe necrotizing pancreatitis develops bluish discoloration of the flanks (Grey Turner sign) and periumbilical bruising (Cullen sign) 48 hours into admission. These physical signs reflect the fact that the pancreas is located in which anatomic compartment?
AIntraperitoneal within the lesser sac only
BSuspended by ventral mesentery in the greater sac
CSecondarily retroperitoneal (except the tail)
DPrimarily retroperitoneal throughout development
EWithin the falciform ligament
Answer: C. The pancreas head and body are secondarily retroperitoneal (SAD PUCKER: Suprarenals, Aorta/IVC, Duodenum D2–D4, Pancreas, Ureters, Colon asc/desc, Kidneys, Esophagus thoracic, Rectum). The tail of the pancreas lies in the splenorenal ligament → intraperitoneal. Retroperitoneal hemorrhage tracks along fascial planes to produce Cullen (periumbilical) and Grey Turner (flank) signs. "Primarily" retroperitoneal = never had mesentery (kidneys, great vessels).
Block 3 — Organ Development · Rotation · Malformations
Block 3 focus: Stomach 90° CW rotation (vagal trunks, lesser sac), liver bud, midgut 270° CCW rotation, and the congenital malformations you must recognize on Step 1.
Question 11 · Stomach Rotation + Vagal Trunks
A general surgeon performing a truncal vagotomy for refractory peptic ulcer disease identifies a nerve running along the anterior surface of the distal esophagus and gastric fundus. Prior to stomach rotation during development, this nerve was located on which side of the stomach?
ALeft side (left vagus → anterior trunk; LARP)
BRight side (right vagus → anterior trunk)
CAnterior (stomach did not rotate)
DMidline (derived from both vagi equally)
EPosterior (right vagus rotates anteriorly)
Answer: A. The stomach rotates 90° clockwise around its long axis (viewed from above), bringing the original left side anterior. The left vagus is carried forward → becomes the anterior trunk. Right vagus → posterior trunk. Mnemonic: LARP (Left Anterior, Right Posterior). This same rotation creates the lesser sac behind the stomach and establishes the greater (dorsal mesentery) and lesser (ventral mesentery) omenta.
Question 12 · Midgut Volvulus — Whirlpool Sign
A previously well 4-week-old infant is brought to the ED with sudden bilious vomiting, lethargy, and bloody stool. Abdominal ultrasound shows the superior mesenteric vein lying to the LEFT of the superior mesenteric artery, with a characteristic "whirlpool" sign on color Doppler. Which embryologic process is MOST likely incomplete?
ARecanalization of the duodenum in weeks 8–10
BObliteration of the vitelline duct
CNormal 270° counterclockwise rotation of the midgut around the SMA
DMigration of the ventral pancreatic bud
EClosure of the pleuroperitoneal canal
Answer: C.Whirlpool sign + SMV left of SMA + bilious emesis in a neonate = malrotation with midgut volvulus. Normal midgut rotation: 90° CCW before herniation (week 6), 180° CCW during return (weeks 10–11) — total 270° CCW around the SMA. Failure leaves a narrow mesenteric base that allows the midgut to twist. This is a surgical emergency → Ladd procedure. Duodenal atresia (A) causes bilious emesis with "double bubble, no distal gas" but no whirlpool.
Question 13 · Double Bubble + Down
A 1-day-old term neonate with upslanting palpebral fissures, single palmar crease, and hypotonia develops bilious vomiting within hours of the first feed. Abdominal X-ray shows a "double bubble" sign with no distal bowel gas. Which embryologic mechanism most likely failed?
AFailed 270° CCW midgut rotation
BFailed recanalization of the duodenum in weeks 8–10
CFailed vagal neural-crest migration
DPersistence of the cloacal membrane
EClosure of the anterior abdominal wall
Answer: B.Double bubble + no distal gas + Down features = duodenal atresia. The duodenum is initially a solid cord; it must recanalize during weeks 8–10. Failure produces a complete obstruction (membrane or atretic segment) proximal to the entry of the bile duct, so vomiting is bilious. Trisomy 21 has the strongest association (~30% of duodenal atresia cases). Annular pancreas can mimic this (also causes obstruction at D2) but typically shows distal gas.
Question 14 · Gastroschisis vs Omphalocele
A newborn is found to have loops of bowel protruding through an abdominal wall defect to the RIGHT of the umbilicus. The exposed bowel is NOT covered by a sac and appears edematous. Karyotype is normal. Maternal AFP was elevated. Which statement best describes this disorder?
AResults from failure of the physiologic midgut herniation to return in week 10–11; commonly associated with trisomy 13/18
BCaused by an in-utero vascular accident (right umbilical vein) producing a paraumbilical wall defect with exposed bowel and no sac
CMidline umbilical defect covered by peritoneum + amnion; associated with Beckwith-Wiedemann
DPersistence of the vitelline duct with intestinal-to-umbilical fistula
ECongenital diaphragmatic hernia through Bochdalek foramen
Answer: B.Right of umbilicus + NO sac + inflamed bowel + normal karyotype = gastroschisis, caused by an in-utero vascular accident (usually right umbilical vein). Not syndromic. Contrast with omphalocele — midline, SAC present, often syndromic (Beckwith-Wiedemann, trisomy 13/18, cardiac anomalies). Both cause elevated maternal AFP. Gastroschisis → urgent surgical closure; bowel edema from amniotic fluid exposure.
Question 15 · Meckel's Diverticulum
A 2-year-old boy presents to clinic with three episodes of painless maroon-colored stools over the past week. He is well-appearing but mildly pale; Hgb is 8.5 g/dL. Abdominal exam is benign; fecal occult blood is strongly positive. Which single diagnostic test is MOST likely to establish the diagnosis?
Answer: C.Painless large-volume rectal bleeding in a toddler = Meckel's diverticulum until proven otherwise. Caused by persistence of the vitelline (omphalomesenteric) duct; contains ectopic gastric mucosa that secretes HCl → ulcerates adjacent ileum. Tc-99m pertechnetate is concentrated by ectopic gastric parietal cells — the Meckel's scan is the gold standard test. Rule of 2s: 2% of population, 2 ft from ileocecal valve, 2 inches long, 2 tissue types, 2% symptomatic, age 2. HIDA is for biliary disease, not GI bleed.
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Total Score
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Block 1 · Endoderm
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Block 2 · Gut Regions
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Block 3 · Rotation
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