Okay, let's tackle this question step by step. The patient is a three-year-old with chronic constipation and abdominal distension for two years. The plain X-ray shows fecal matter and distended bowel loops. Then a barium enema reveals a transition zone at the recto-sigmoid junction with reversal of the recto-sigmoid ratio. The key here is the transition zone and the reversed ratio.

First, I need to recall the conditions that cause such findings. Hirschsprung's disease comes to mind because it's a congenital aganglionic megacolon. The absence of ganglion cells in the myenteric plexus of the distal colon leads to obstruction, causing proximal dilation. The transition zone between the aganglionic (narrow) and ganglionic (dilated) segments is a hallmark. The recto-sigmoid ratio reversal would mean the rectum is narrower than the sigmoid, which is the opposite of normal.

Other options might include chronic constipation from other causes, like functional constipation, but that doesn't have a structural transition zone. Anorectal malformations usually present with different symptoms, like absence of meconium. Colonic atresia is a neonatal issue, not this late. So the correct answer is Hirschsprung's disease.

Now, the explanation needs to cover the core concept of Hirschsprung's, why the correct answer fits with the transition zone and ratio reversal, and why the other options are incorrect. Also, a clinical pearl about the barium enema findings and the importance of rectal biopsy for confirmation.

**Core Concept**
The question tests recognition of **Hirschsprung’s disease**, a congenital aganglionic megacolon caused by absence of ganglion cells in the myenteric plexus of the distal colon. The key diagnostic clue is a **transition zone** between aganglionic (narrow) and ganglionic (dilated) bowel, with **recto-sigmoid ratio reversal** (rectum narrower than sigmoid colon), confirmed by imaging or rectal biopsy.

**Why the Correct Answer is Right**
In Hirschsprung’s disease, the aganglionic segment (typically rectosigmoid) fails to relax, causing functional obstruction. Proximal bowel dilates ("megacolon"), while the affected segment remains narrow. The **barium enema** shows a **transition zone** at the rectosigmoid junction and **recto-sigmoid ratio reversal** (rectal diameter < sigmoid diameter). Histologically, absence of **Meissner’s and Auerbach’s plexuses** in the submucosa and muscularis confirms diagnosis. **Why Each Wrong Option is Incorrect** **Option A:** *Chronic functional constipation* lacks structural abnormalities like transition zones or ratio reversal. **Option B:** *Colonic atresia* presents in neonates with bilious vomiting and absence of meconium, not chronic constipation. **Option C:** *Anorectal malformations* (e.g., imperforate anus) present with no anal opening or abnormal perineal anatomy, not imaging findings of a transition zone. **Clinical Pearl / High-Yield Fact** Hirschsprung’s disease is **diagnosed definitively by rectal biopsy** showing absence of ganglion cells. Bar

✓ Correct Answer: C. Hirschsprung's disease