## **Core Concept**
Hirschsprung disease is a congenital condition characterized by the absence of ganglion cells in the distal bowel, leading to a functional obstruction. The diagnosis involves demonstrating the absence of these cells in the affected segment. The condition primarily affects the distal colon and rectum.

## **Why the Correct Answer is Right**
The correct answer, **Rectal Biopsy**, is considered the gold standard for diagnosing Hirschsprung disease. This procedure involves taking a tissue sample from the rectum, which is then examined for the presence or absence of ganglion cells. The absence of these cells confirms the diagnosis. Rectal biopsy is highly specific and sensitive for this condition, making it the best investigation.

## **Why Each Wrong Option is Incorrect**
- **Option A: Ultrasound**: While ultrasound can show signs suggestive of Hirschsprung disease, such as a transition zone between the dilated and undilated bowel, it is not definitive for diagnosing the absence of ganglion cells.
- **Option B: Barium Enema**: Barium enema can demonstrate a transition zone and other features suggestive of Hirschsprung disease but does not provide a histological diagnosis.
- **Option C: CT Scan**: CT scan is not typically used for diagnosing Hirschsprung disease, especially in children, due to radiation exposure concerns and its inability to directly assess for ganglion cells.
- **Option D: MRI**: MRI is not routinely used for the diagnosis of Hirschsprung disease. It does not offer advantages over rectal biopsy in terms of diagnosing the condition.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that Hirschsprung disease is often suspected in infants who fail to pass meconium within the first 48 hours of life. A rectal biopsy is crucial for confirming the diagnosis by demonstrating the absence of ganglion cells.

## **Correct Answer:** . Rectal Biopsy