**Core Concept:** Bilious vomiting in a newborn can indicate various conditions affecting the gastrointestinal (GI) tract or liver. An X-ray abdomen can reveal key findings to narrow down the diagnosis.

**Why the Correct Answer is Right:**
The correct answer is **C. Meconium-stained gastric contents**. Meconium is the first stool passed by an infant, usually within the first 24 hours of life. In the context of bilious vomiting, meconium-stained vomitus indicates that the vomiting is occurring before the infant has had an opportunity to pass meconium stool. This may be due to various conditions, including gastric outlet obstruction (e.g., pyloric stenosis) or dysfunction of the gastroesophageal sphincter (e.g., gastroesophageal reflux disease).

**Why Each Wrong Option is Incorrect:**
A. **Option A (Meconium passage)** - This is a separate condition where the infant passes meconium stool, not bilious vomiting.
B. **Option B (Perforation)** - Perforation typically presents with signs of peritonitis, not just bilious vomiting.
D. **Option D (Ascites)** - Ascites is a condition involving fluid in the peritoneal cavity, not bilious vomiting.

**Clinical Pearl:**
X-ray abdomen can be a helpful initial investigation to evaluate for the presence of meconium or air, which can guide further diagnostic tests and management decisions.

**Correct Answer:** C. Meconium-stained gastric contents