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?

The infant's condition occurred several weeks after birth because of the hypertrophy of pyloric smooth muscle. Pyloric stenosis has features of multifactorial inheritance with a "threshold of liability", above which the disease is manifested when more genetic risks are present, such as family history and twin gestation. The incidence in males is 1 in 200 and females are 1 in 1000, reflecting the fact that more risks must be present in females for the disease to occur. Annular pancreas is a rare anomaly that can also cause obstruction of the duodenum and has a variable age of onset, but a palpable mass would not be expected. Tracheoesophageal fistula, diaphragmatic hernia, and duodenal atresia are serious conditions that are manifested at birth and are often associated with multiple anomalies. Pyloric stenosis is an isolated condition that typically occurs without other anomalies.