Inflammatory Bowel Disease USMLE Step 1 Practice Question

A full-term newborn presents with abdominal distension and failure to pass meconium beyond 48 hours of life. Vital signs show HR 145/min, RR 48/min, temperature 37.2°C. Physical examination reveals a distended abdomen; rectal examination produces explosive passage of stool and flatus. Rectal suction biopsy demonstrates absence of ganglion cells in the submucosa and muscularis propria. Contrast enema shows a transition zone. The infant has not received prior antibiotics. Which diagnosis best explains these findings?