Which condition is most likely diagnosed in a 4-week-old infant who has projectile vomiting after feeding, appears well after vomiting, and has a palpable mass in the abdomen?

The condition most likely diagnosed in a 4-week-old infant who exhibits projectile vomiting after feeding, appears well after vomiting, and has a palpable abdominal mass is pyloric stenosis. This congenital condition occurs when the pylorus, which is the opening from the stomach to the small intestine, becomes narrowed.

The key features presented in the scenario are characteristic of pyloric stenosis: the projectile vomiting is due to the obstruction preventing food from passing into the small intestine, and the infant's appearance of being well after vomiting is typical because they usually can tolerate feeds between episodes. The palpable abdominal mass often described is the "olive sign," which is the hypertrophied pylorus that can be felt in the right upper quadrant of the abdomen.

In contrast, conditions like milk allergy might cause vomiting, but it is usually associated with other symptoms like rash, irritability, or diarrhea rather than a palpable mass. Intussusception typically presents with abdominal pain, "currant jelly" stools, and may not be described as well tolerated between episodes. Gastroesophageal reflux disease (GERD) can result in vomiting but usually does not include projectile vomiting or a palpable abdominal mass, and infants with GERD generally exhibit different feeding behaviors and additional symptoms.