## **Core Concept**
The clinical presentation of a newborn girl who has not passed meconium within 48 hours, along with symptoms of abdominal distention and vomiting, suggests a possible diagnosis of **intestinal obstruction**. This condition can be caused by various factors, including **meconium ileus**, **intestinal atresia**, or **Hirschsprung's disease**. The initial investigation of choice should quickly and effectively help diagnose or rule out these conditions.

## **Why the Correct Answer is Right**
The correct answer, **X-ray abdomen**, is the initial investigation of choice because it is a rapid, non-invasive, and readily available imaging technique. It can help identify signs suggestive of intestinal obstruction, such as **dilated bowel loops**, **air-fluid levels**, or **absence of gas in the rectosigmoid region**. These findings can guide further management and potentially point towards conditions like Hirschsprung's disease or mechanical obstruction.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Ultrasound abdomen, while useful in certain pediatric conditions, is not the first-line imaging modality for suspected intestinal obstruction in newborns due to its operator-dependent nature and lesser availability compared to X-rays.
- **Option B:** CT abdomen is not typically used in newborns as an initial investigation due to radiation exposure concerns and the availability of other diagnostic modalities that are less harmful.
- **Option C:** MRI abdomen, although useful for detailed evaluation of soft tissues, is not practical as an initial investigation in acute settings due to its longer examination time, higher cost, and lesser availability.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that **Hirschsprung's disease** often presents with a history of constipation, abdominal distention, and failure to pass meconium within the first 48 hours of life. On an abdominal X-ray, one might see signs of intestinal obstruction. A **rectal biopsy** is often diagnostic for Hirschsprung's disease, showing an absence of ganglion cells.

## **Correct Answer:** . **X-ray abdomen**