## **Core Concept**
The diagnosis of congenital megacolon, also known as Hirschsprung's disease, involves identifying the absence of ganglion cells in the distal bowel, leading to a functional obstruction. This condition is characterized by a failure of the neural crest cells to migrate to the distal parts of the colon during fetal development.

## **Why the Correct Answer is Right**
The correct answer, **Rectal biopsy**, is the gold standard for diagnosing Hirschsprung's disease. This procedure involves taking a tissue sample from the rectum to examine for the presence or absence of ganglion cells. The absence of these cells confirms the diagnosis. Rectal biopsy is crucial because it directly assesses the presence of ganglion cells in the submucosal and myenteric plexus of the rectum.

## **Why Each Wrong Option is Incorrect**
- **Option A: Ultrasound abdomen** - While ultrasound can show signs suggestive of intestinal obstruction or dilated bowel loops, it cannot confirm the absence of ganglion cells.
- **Option B: Barium enema** - This radiographic study can show a transition zone between the dilated and undilated bowel, suggestive of Hirschsprung's disease, but it is not definitive for diagnosing the absence of ganglion cells.
- **Option D: Manometry** - Anorectal manometry can assess the function of the anal sphincter and rectal inhibitory reflex, which is often absent in Hirschsprung's disease. However, it is not as definitive as a rectal biopsy for confirming the diagnosis.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that Hirschsprung's disease often presents with constipation, abdominal distension, and vomiting in newborns. A rectal biopsy demonstrating the absence of ganglion cells is diagnostic. Remember, the condition is due to a failure of neural crest cell migration, leading to aganglionosis.

## **Correct Answer:** . Rectal biopsy