**Core Concept**
Bilious vomiting is a clinical sign that indicates an obstruction in the gastrointestinal tract beyond the ampulla of Vater, where bile enters the duodenum. The presence of bile in vomit suggests a distal intestinal obstruction, which is a medical emergency.

**Why the Correct Answer is Right**
Atresia of the 3rd part of the duodenum is a congenital anomaly characterized by a complete obstruction of the duodenum at the level of the ampulla of Vater. This obstruction prevents the normal flow of bile into the small intestine, resulting in bilious vomiting. The 3rd part of the duodenum is the most common site for duodenal atresia, which is a classic cause of bilious vomiting in neonates.

**Why Each Wrong Option is Incorrect**
**Option A:** Pyloric stenosis typically presents with non-bilious projectile vomiting due to a narrowing of the pyloric channel, which is a proximal obstruction. The absence of bile in the vomit is characteristic of this condition.
**Option B:** Esophageal atresia presents with non-bilious vomiting due to an obstruction in the esophagus, which is a proximal obstruction. The vomit is usually clear or contains saliva.
**Option D:** Malrotation of the gut can present with bilious vomiting, but it is not a guaranteed presentation. The vomiting may be non-bilious initially, and the presence of bile depends on the level of obstruction and the degree of malrotation.

**Clinical Pearl / High-Yield Fact**
In neonates, bilious vomiting is a red flag symptom that requires immediate evaluation for a distal intestinal obstruction. Duodenal atresia is a classic cause of bilious vomiting, and its presence should be suspected in any neonate with this symptom.

**✓ Correct Answer: C. Atresia of the 3rd part of the duodenum**