A newborn with Down syndrome has constipation. Rectal examination shows no stool in the rectal ampulla. The proximal rectosigmoid is dilated on radiograph. The mechanism most likely responsible for the patient's disease is

Down syndrome patients have an increased incidence of Hirschsprung disease (10% of all cases), which commonly presents with inability to pass meconium after 24 hours. * Hirschsprung disease, or congenital megacolon, is a deficiency of ganglion cells in Meissner submucosal plexus and Auerbach myenteric plexus, which causes an absence of peristalsis, * Approximately 90% of cases are restricted to the rectum. The bowel proximal to the defect is dilated because of the inability of the peristaltic wave to push stool beyond the aganglionic segment, resulting in a functional rather than an anatomical obstruction. * Megaesophagus and megaloureter may also be present in these patients. The diagnosis of Hirschsprung disease is secured by obtaining rectal biopsies and noting the absence of ganglion cells. * Ganglion cells are present in the proximal dilated bowel. Complications include entero colitis, toxic megacolon, and the potential for perforation.