## **Core Concept**
Hirschsprung's disease is a congenital condition characterized by the absence of ganglion cells in the distal bowel, leading to a functional obstruction. This condition primarily affects the large intestine (colon) and is due to an abnormal development of the enteric nervous system. The enteric nervous system is crucial for regulating bowel movements.

## **Why the Correct Answer is Right**
The correct answer, **C. Absence of ganglion cells**, is right because Hirschsprung's disease is fundamentally caused by the failure of neural crest cells to migrate to the distal parts of the colon during fetal development. These neural crest cells give rise to the ganglion cells of the enteric nervous system. Without ganglion cells, the affected segment of the colon cannot relax and pass stool, leading to obstruction.

## **Why Each Wrong Option is Incorrect**
- **Option A: Overgrowth of intestinal mucosa** - This is not a recognized cause of Hirschsprung's disease. The condition is not related to overgrowth but rather to a developmental failure.
- **Option B: Increased muscle tone** - While increased muscle tone may be a feature in the affected segment of the colon due to the lack of ganglion cells, it is not the primary cause of Hirschsprung's disease.
- **Option D: Infiltration of eosinophils** - Eosinophilic infiltration is not associated with the pathogenesis of Hirschsprung's disease.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that Hirschsprung's disease often presents in infancy with symptoms like constipation, abdominal distension, and vomiting. A definitive diagnosis is made by rectal biopsy showing the absence of ganglion cells. A classic exam trap is to consider conditions like intestinal obstruction or infection; however, the hallmark is the absence of ganglion cells.

## **Correct Answer: C. Absence of ganglion cells.**