What is a common cause of dehydration in infants presenting with vomiting and diarrhea after a viral illness?

Gastroenteritis is a common cause of dehydration in infants, especially when they present with vomiting and diarrhea following a viral illness. This condition typically arises from a viral infection that inflames the stomach and intestines, leading to an increased loss of fluids and electrolytes through vomiting and loose stools. Infants are particularly vulnerable to dehydration due to their smaller body size and higher proportion of body water compared to adults. When they experience gastroenteritis, the rapid fluid loss can quickly lead to significant dehydration, making monitoring and rehydration essential components of their care.

In contrast, while allergic reactions can cause gastrointestinal symptoms, they are less directly associated with dehydration following viral illnesses. Pyloric stenosis and intestinal obstruction can lead to vomiting, but they are less common and often present with specific symptoms such as projectile vomiting or severe abdominal distension, respectively, rather than following a viral illness. Thus, gastroenteritis is the most likely and common cause in such scenarios.