What is the primary symptom of pyloric stenosis in infants?

The primary symptom of pyloric stenosis in infants is projectile vomiting. This condition occurs when the pylorus, which is the opening from the stomach into the small intestine, becomes narrowed, obstructing the passage of food. As a result, when an infant eats, the stomach contracts to push the food through the narrowed pylorus, leading to forceful vomiting that can often shoot several feet away. This symptom typically presents when the infant is fed, occurring most commonly in infants aged 3 to 12 weeks.

In contrast, abdominal bloating may occur in various gastrointestinal conditions but is not the hallmark symptom of pyloric stenosis. Weight gain is generally not associated with this condition since the vomiting can lead to dehydration and inadequate caloric intake. Excessive drooling may suggest other conditions such as teething or gastrointestinal issues but does not specifically indicate pyloric stenosis. The distinctive nature of the projectile vomiting in pyloric stenosis sets it apart as the primary symptom for diagnosis.