A 3-week-old boy is brought to the physician by his parents, who report that he vomits forcefully immediately after nursing. Physical examination reveals an “olive-like” palpable mass and visible peristaltic movements within the infant’s abdomen. What is the most likely cause of projectile vomiting in this infant?

**Core Concept**
Forceful vomiting in an infant, accompanied by an "olive-like" palpable mass and visible peristaltic movements, is suggestive of intestinal obstruction. The presence of these symptoms, particularly in a 3-week-old infant, raises the suspicion of a congenital anomaly.

**Why the Correct Answer is Right**
The symptoms described are classic for pyloric stenosis, a condition characterized by hypertrophy of the pyloric muscle, leading to obstruction of the gastric outlet. This results in forceful, projectile vomiting after feeding, often accompanied by visible peristaltic movements. The "olive-like" palpable mass is due to the thickened pyloric muscle.

**Why Each Wrong Option is Incorrect**
* **Option A:** Intussusception is a condition where a segment of the intestine invaginates into another segment, leading to obstruction and potentially ischemia. While it can cause vomiting, the presence of an "olive-like" palpable mass and visible peristaltic movements is not typical.
* **Option B:** Duodenal atresia is a congenital anomaly characterized by a complete obstruction of the duodenum. Infants with duodenal atresia typically present with bilious vomiting, not projectile vomiting.
* **Option C:** Hirschsprung's disease is a congenital condition characterized by the absence of ganglion cells in the distal colon, leading to functional obstruction. While it can cause vomiting, the symptoms are typically constipation and abdominal distension, not projectile vomiting.

**Clinical Pearl / High-Yield Fact**
In infants with suspected pyloric stenosis, a "string sign" on barium studies is a classic finding, where the barium column stops abruptly at the level of the pylorus.

**Correct Answer: C. Hirschsprung's disease is a congenital condition characterized by the absence of ganglion cells in the distal colon, leading to functional obstruction. While it can cause vomiting, the symptoms are typically constipation and abdominal distension, not projectile vomiting.

✓ Correct Answer: B. Congenital pyloric stenosis