Embryology USMLE Step 1 Practice Question
A 9-day-old male infant is admitted with bilious vomiting, abdominal distension, and inability to pass meconium. His mother reports that vomiting began 6 hours after his first feeding. On examination, the infant is irritable with HR 148 bpm, RR 40/min, BP 62/38 mmHg, and normal temperature. The abdomen is diffusely distended without peritoneal signs. Abdominal radiograph shows the classic "double bubble" sign with air in the stomach and proximal duodenum, but little distal bowel gas. Upper gastrointestinal contrast study reveals an abrupt transition point distal to the ampulla of Vater, and fluoroscopy shows the duodenum spiraling around a fixed vascular pedicle in the midline. Which of the following embryologic events best explains this presentation?
Answer choices
- AFailure of the mesentery to fuse with the posterior peritoneum, resulting in a mobile cecum
- BPersistence of the omphalomesenteric duct with formation of a Meckel diverticulum
- CIncomplete rotation of the midgut around the superior mesenteric artery, with the duodenum encircled by anomalous peritoneal bandsCorrect answer
- DFailure of the dorsal pancreatic bud to fuse with the ventral bud, creating pancreatic tissue that compresses the duodenum
- EFailure of the hepatic flexure to rotate clockwise, resulting in ectopic gastric mucosa
- FAgenesis of the annular pancreas, allowing the duodenum to rotate excessively
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