**Core Concept:** Hirschsprung disease is a congenital disorder characterized by the absence of ganglion cells in the distal bowel, leading to impaired peristalsis and colonic obstruction. This results in chronic constipation, rectal prolapse, and distension of the colon.

**Why the Correct Answer is Right:** Hirschsprung disease is the most appropriate diagnosis in this scenario due to the presence of fecal matter and distended bowel loops on abdominal radiograph, and the transition zone and reversed recto-sigmoid ratio on barium enema study. These findings are consistent with the pathophysiology of Hirschsprung disease, characterized by the absence of ganglion cells in the distal colon, leading to impaired peristalsis and obstruction.

**Why Each Wrong Option is Incorrect:**

A. Hirschsprung disease is not specific to children under three years old, but it can occur in older children and adults as well.
B. While constipation is present, the absence of abdominal distension and the presence of rectal prolapse favor other conditions like megacolon or fecal impaction.
C. Although megacolon can present with constipation and distension, the absence of transition zone and reversed recto-sigmoid ratio on barium enema is inconsistent with this diagnosis.
D. Feces impaction can present with constipation and distension, but the presence of transition zone and reversed recto-sigmoid ratio on barium enema is inconsistent with this diagnosis.

**Clinical Pearl:** Hirschsprung disease is a crucial differential diagnosis in children presenting with chronic constipation and abdominal distension, especially when associated with rectal prolapse or failure to pass stools despite apparent rectal evacuation attempts. Barium enema is a valuable diagnostic tool to identify the transition zone and reversed recto-sigmoid ratio, which is characteristic of this disorder.

**Correct Answer:** C. Hirschsprung disease