**Core Concept**
The "double bubble" sign on ultrasound scan is indicative of duodenal obstruction, a condition where the normal flow of intestinal contents is blocked at the level of the duodenum, the first portion of the small intestine. This obstruction can be caused by various factors, including congenital anomalies, volvulus, or external compression.

**Why the Correct Answer is Right**
The symptoms described, including persistent vomiting, failure to gain weight, and abdominal distension, are consistent with duodenal obstruction. The "double bubble" sign on ultrasound scan confirms this diagnosis by showing the presence of two distinct fluid-filled structures in the abdomen, representing the stomach and the proximal duodenum. This sign is characteristic of duodenal atresia, a congenital anomaly where there is a complete obstruction of the duodenum due to the absence of a portion of the intestine. The infant's constant hunger and crying are also indicative of the discomfort and malnutrition caused by this obstruction.

**Why Each Wrong Option is Incorrect**
**Option A:** Gastroesophageal reflux disease (GERD) typically presents with regurgitation of feeds, irritability, and failure to thrive, but it would not cause a "double bubble" sign on ultrasound scan.
**Option B:** Intestinal obstruction due to volvulus or external compression may present with similar symptoms, but the "double bubble" sign is more specific for duodenal atresia.
**Option C:** Hirschsprung's disease is a congenital anomaly of the colon, characterized by the absence of ganglion cells in the distal colon, leading to constipation and intestinal obstruction. It would not cause a "double bubble" sign on ultrasound scan.
**Option D:** pyloric stenosis presents with projectile vomiting, but it would not cause a "double bubble" sign on ultrasound scan.

**Clinical Pearl / High-Yield Fact**
The "double bubble" sign on ultrasound scan is a classic radiographic finding in duodenal atresia, a congenital anomaly that requires prompt surgical intervention to prevent complications such as malnutrition, dehydration, and sepsis.

**Correct Answer:** C.