What is the most likely diagnosis for a 5-month-old infant with stable weight and frequent vomiting after feeds?

The most likely diagnosis for a 5-month-old infant who exhibits stable weight along with frequent vomiting after feeds aligns with gastroesophageal reflux (GER). In infants, GER is a common condition characterized by the backward flow of stomach contents into the esophagus, leading to symptoms such as vomiting or spitting up after feeding.

Key indicators in this scenario include the infant's stable weight despite the vomiting, which suggests that the child is still receiving adequate nutrition and is likely thriving despite the reflux episodes. This is typical in many cases of GER, where infants may appear well-nourished and maintain weight, as the condition can often be managed effectively without significant impact on growth if the vomiting is not excessive or leading to dehydration.

In contrast, conditions like pyloric stenosis generally present with projectile vomiting and possible weight loss due to the obstruction in the stomach, leading to poor nutrition. Failure to thrive would usually involve a significant drop in weight or growth percentiles, which does not match the infant's profile in this case. Pneumonia primarily manifests with respiratory symptoms like cough or difficulty breathing rather than vomiting after feeds. Therefore, the combination of stable weight and frequent postprandial vomiting strongly supports the diagnosis of gastroesophageal reflux.