## **Core Concept**
The core concept here relates to the diagnosis and management of **delayed passage of meconium** in a newborn. Normally, a newborn passes meconium within the first 24 hours of life. Delayed passage of meconium beyond 48 hours can indicate an underlying condition such as **Hirschsprung's disease**, which is characterized by the absence of ganglion cells in the distal bowel, leading to a functional obstruction.

## **Why the Correct Answer is Right**
The diagnostic procedure of choice for Hirschsprung's disease, which could be a cause of delayed passage of meconium, involves a **rectal biopsy**. This procedure allows for the histological examination of the rectal tissue to assess for the presence or absence of ganglion cells. The absence of these cells is diagnostic of Hirschsprung's disease. This method is considered the gold standard for diagnosis.

## **Why Each Wrong Option is Incorrect**
- **Option A:** While an abdominal X-ray can show signs suggestive of intestinal obstruction, such as dilated loops of bowel, it does not provide a definitive diagnosis of Hirschsprung's disease or other causes of delayed meconium passage.
- **Option B:** A barium enema can provide clues towards the diagnosis by showing a transition zone between the normal and aganglionic bowel, but it is not as definitive as a rectal biopsy.
- **Option C:** Ultrasound of the abdomen is not typically used as a primary diagnostic tool for Hirschsprung's disease or delayed passage of meconium.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that **Hirschsprung's disease** often presents with constipation, abdominal distension, and failure to pass meconium within the first 48 hours of life. A rectal biopsy is essential for confirming the diagnosis by demonstrating the absence of ganglion cells.

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