**Question:** A newborn baby has not passed meconium for 48 hours since birth. She has vomiting and distension of abdomen. The most appropriate investigation for evaluation would be:

A. Abdominal X-ray
B. Abdominal ultrasound
C. Complete blood count (CBC)
D. Stool culture and sensitivity

**Correct Answer:** D. Stool culture and sensitivity

**Core Concept:**
In neonates, meconium is the first stool passed after birth, usually appearing within the first 24-48 hours. Vomiting and abdominal distension in a neonate indicate potential gastrointestinal (GI) tract issues. The most appropriate investigation to identify the cause would be a stool culture and sensitivity test.

**Why the Correct Answer is Right:**
Meconium impaction is a rare but serious cause of neonatal intestinal obstruction. It can lead to progressive abdominal distension, vomiting, and poor feeding. Meconium impaction should be considered when meconium passage is delayed and associated with abdominal symptoms. A stool culture and sensitivity test helps identify the possible pathogens causing the obstruction, guiding targeted antibiotic therapy.

**Why Each Wrong Option is Incorrect:**
A. Abdominal X-ray (A): While an abdominal X-ray can be helpful in detecting intestinal obstruction, it does not identify the causative pathogen. In this context, a culture and sensitivity test is more appropriate.

B. Abdominal ultrasound (B): An abdominal ultrasound can assess intestinal obstruction, but it does not directly identify the causative pathogen. A culture and sensitivity test is more suitable for this purpose.

C. Complete blood count (C): A CBC is a general blood test that provides information about the baby's overall health but does not address the specific cause of the symptoms. A culture and sensitivity test is more pertinent to the issue.

**Clinical Pearl:**
The neonate presenting with abdominal distension, vomiting, and delayed meconium passage should raise suspicion for meconium impaction, and requesting a culture and sensitivity test will help identify the causative pathogen, guiding appropriate antibiotic therapy.