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This patient's contrast enema shows an abrupt transition zone between the narrow rectosigmoid segment and the dilated descending colon (ie, megacolon).  This finding is characteristic of Hirschsprung disease (HD), a congenital disorder caused by disrupted migration of neural crest cells to the intestinal wall during embryogenesis.  Neural crest cells normally migrate in a craniocaudal direction and give rise to ganglion cells of the submucosal (Meissner) and myenteric (Auerbach) plexus in the colon.  Abnormal migration results in the absence of parasympathetic innervation of the distal rectosigmoid segment.

Without parasympathetic innervation, the aganglionic colonic segment is unable to relax (ie, chronically contracted and narrowed), making stool difficult to pass.  Presentation in the neonatal period with delayed passage of meconium (>48 hr after birth) is classic, but infants with short aganglionic segments may have fussiness due to infrequent stooling/chronic constipation.  The buildup of stool causes abdominal distension and dilation of the histologically normal colon proximal to the obstruction.

Diagnosis of HD is confirmed by the absence of ganglia on rectal suction biopsy.  Treatment is with surgical resection of the aganglionic segment.

(Choice A)  Paravertebral ganglia lie adjacent to the vertebrae and are involved in sympathetic nervous signaling; paravertebral ganglia are not affected by HD.

(Choice B)  Disruption of small bowel recanalization is the cause of duodenal atresia, which presents in the first 1-2 days of life with bilious emesis.  Imaging would show a dilated stomach and proximal duodenum.  In addition, microcolon (not a dilated colon) would be expected on barium enema.

(Choice C)  Malrotation (incomplete rotation of the intestines) often presents in infancy with bilious vomiting and abdominal distension.  Contrast studies show right-sided jejunal loops and may reveal a corkscrew duodenum if malrotation is complicated by volvulus (intestinal torsion).  Colonic dilation would not be seen.

(Choice D)  Superior mesenteric artery (SMA) syndrome causes bowel obstruction due to external duodenal compression by the SMA.  Abdominal pain and distension are typical, but barium enema would not show a rectosigmoid transition zone; instead, oral contrast studies would show absence of barium beyond the duodenum.

Educational objective:
Hirschsprung disease is characterized by absent submucosal and myenteric ganglion cells in the distal colon.  Contrast enema classically shows a narrowed aganglionic segment (most commonly rectosigmoid) with an abrupt transition zone to the dilated, histologically normal proximal colon.