A three-year-old male child presents with a history of constipation and abdominal distension for the last two years. The plain radiograph of the abdomen reveals fecal matter containing distended bowel loops. A barium enema study done subsequently shows a transition zone at the recto-sigmoid junction with reversal of recto-sigmoid ratio. The most probable diagnosis is -

Hirschsprung's disease usually presents shortly after birth but sometimes the onset can be delayed. All the other features in this patient confirms the diagnosis of Hirschspring's disease. The contrast enema in case of Hirschsprung's disease demonstrates. Aganglionic segment that appears narrow in comparison with the dilated proximal bowel. This zone of the intestine with proximal dilated intestine along with caudal narrow aganglionic segment is the transition zone. Normally the rectum is wider than the rest of the colon (except caecum) and when the rectum is seen to be narrower than the proximal colon. The diagnosis is Hirschsprung's disease.