## **Core Concept**
The question tests the understanding of a common congenital condition affecting the gastrointestinal tract, specifically the large intestine, leading to obstructive symptoms. The key findings here include chronic constipation, abdominal distension, and specific radiographic features.

## **Why the Correct Answer is Right**
The clinical presentation and imaging findings are classic for **Hirschsprung's disease**. This condition is characterized by the absence of ganglion cells in the distal bowel, leading to a functional obstruction. The barium enema findings of a transition zone at the recto-sigmoid junction with a reversal of the recto-sigmoid ratio (where the rectum appears narrower than the sigmoid colon) are highly suggestive of Hirschsprung's disease. This condition typically presents in infancy or early childhood with symptoms like constipation, abdominal distension, and failure to pass meconium.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While other conditions can cause constipation and abdominal distension, the specific radiographic findings mentioned are not typical for conditions like functional constipation or gastrointestinal infections.
- **Option B:** This option is not provided, but any condition not specifically related to the described pathophysiology and imaging findings would be incorrect.
- **Option D:** Similarly, without a specific option provided, we acknowledge that any alternative diagnosis not aligning with the clinical and radiographic presentation of Hirschsprung's disease would be incorrect.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that **Hirschsprung's disease** often presents with a history of delayed passage of meconium (>48 hours after birth) and can be diagnosed through a rectal biopsy showing the absence of ganglion cells. The condition requires surgical intervention for definitive management.

## **Correct Answer:** C. Hirschsprung's disease.