**Core Concept**
Malrotation is a congenital anomaly of the intestinal tract where the midgut fails to return to its normal anatomic position during fetal development. This results in an abnormal intestinal rotation and fixation, leading to potential complications such as intestinal obstruction, volvulus, and ischemia.

**Why the Correct Answer is Right**
Bilious vomiting is a classic feature of intestinal obstruction due to malrotation. The obstruction prevents normal intestinal contents from passing through, causing a buildup of bile and gastric secretions. This leads to the characteristic bilious vomiting seen in patients with malrotation. The obstruction can occur at any level of the small intestine, from the duodenojejunal junction to the ileum.

**Why Each Wrong Option is Incorrect**
**Option A:** Mass abdomen is not a specific feature of malrotation. While abdominal distension may occur due to intestinal obstruction, a palpable abdominal mass is not a characteristic finding.

**Option B:** Bleeding PR (per rectum) is not typically associated with malrotation. While intestinal ischemia can occur in severe cases, it is not a common presentation.

**Option D:** Hematemesis (vomiting blood) is not a typical feature of malrotation. While gastrointestinal bleeding can occur due to ischemia or other complications, it is not a characteristic presentation.

**Clinical Pearl / High-Yield Fact**
The "double bubble" sign on abdominal X-ray is a classic radiographic finding in intestinal obstruction due to malrotation. This sign is characterized by two air-filled structures in the upper abdomen, representing the dilated stomach and duodenum.

**Correct Answer: C. Bilious vomiting**