A 4-day-old male infant vomits the contents of his stomach, but the vomitus does not appear to contain bile. The baby is obviously distressed and makes sucking movements of his lips in response to offerings to suckle by his mother or of the bottle, but he is failing to thrive. Which of the following conditions will best explain the symptoms?

With hypertrophy of the pyloric sphincter and the associated narrowing of the opening, there is projectile vomiting of stomach contents, but without bile, because bile enters the duodenum distal to the pyloric constriction. Duodenal atresia, like duodenal stenosis, causes vomiting of stomach contents and bile. Vomiting begins soon after birth in cases of atresia; vomiting due to stenosis does not begin necessarily immediately after birth and can occur days after delivery. Lack of weight gain is due to constant vomiting. An atrophied gastric fundus would not produce the signs seen here. A tracheoesophageal fistula would not cause vomiting of stomach contents and bile because it is a defect of the respiratory system and occurs proximal to the site at which bile is added to intestinal contents.