A 23-year-old woman, G2, P1, gave birth at term to a boy of normal weight and length following an uncomplicated pregnancy. The infant initially did well, but at 6 weeks, he began feeding poorly for 1 week, and his mother noticed that much of the milk he ingested was forcefully vomited within 1 hour. Now, on physical examination, the infant is afebrile, and there are no external anomalies. A midabdominal mass is palpable. Bowel sounds are active. The medical history indicates that both the mother and her first child had the same illness during infancy. Which of the following conditions is most likely to explain these findings?

**Core Concept**
The question describes a case of an infant presenting with projectile vomiting, a palpable abdominal mass, and a history of similar illness in the mother and her first child. This clinical presentation is suggestive of a congenital anomaly affecting the gastrointestinal system, specifically a condition that involves an abnormal connection between the stomach and the small intestine.

**Why the Correct Answer is Right**
The condition that best explains these findings is a **Gastrointestinal duplication cyst**, specifically a **gastric duplication cyst**. This congenital anomaly is characterized by the presence of a cystic structure in the gastrointestinal tract, which in this case is connected to the stomach. The cyst can cause symptoms such as projectile vomiting, abdominal mass, and feeding difficulties due to obstruction or compression of surrounding structures. The fact that both the mother and her first child had similar symptoms suggests a possible genetic component or familial predisposition to this condition.

**Why Each Wrong Option is Incorrect**
**Option A:** **Meckel's diverticulum** is a congenital anomaly of the small intestine, characterized by a small bulge in the wall of the ileum. While it can cause gastrointestinal bleeding or obstruction, it is not typically associated with a palpable abdominal mass or projectile vomiting.

**Option B:** **Hirschsprung's disease** is a congenital condition affecting the large intestine, characterized by the absence of ganglion cells in the distal colon and rectum. Symptoms include constipation, abdominal distension, and failure to pass meconium. It does not typically present with projectile vomiting or a palpable abdominal mass.

**Option C:** **Gastroesophageal reflux disease (GERD)** is a common condition in infants, characterized by the reflux of stomach contents into the esophagus. While it can cause symptoms such as vomiting and feeding difficulties, it is not typically associated with a palpable abdominal mass.

**Option D:** **Intussusception** is a condition characterized by the invagination of one segment of the intestine into another. Symptoms include abdominal pain, vomiting, and bloody stools. While it can cause a palpable abdominal mass, it is not typically associated with a history of similar illness in the mother and her first child.

**Clinical Pearl / High-Yield Fact**
This case highlights the importance of considering congenital anomalies in the differential diagnosis of infantile vomiting and abdominal masses. A high index of suspicion and a thorough family history can aid in the diagnosis of these conditions.

**Correct Answer:** C. Gastrointestinal duplication cyst.

✓ Correct Answer: D. Pyloric stenosis