In infants with pyloric stenosis, which of the following is least likely to be observed?

Pyloric stenosis in infants is characterized by the narrowing of the pylorus, which can lead to severe projectile vomiting. This vomiting can cause significant electrolyte imbalances and metabolic disturbances.

Hypernatremia is typically a result of dehydration, which can occur in any infant who is not receiving sufficient hydration or experiencing excessive loss of fluids. In the case of pyloric stenosis, while infants may experience some degree of dehydration due to vomiting, the most prominent electrolyte imbalances observed are usually related to significant fluid loss leading to hypochloremia (low chloride levels) and hypokalemia (low potassium levels) because the contents of the stomach, which include these electrolytes, are being continuously expelled.

Additionally, metabolic alkalosis can develop due to the loss of gastric acid from the vomiting, leading to an increase in blood pH. Therefore, while hypernatremia can occur, it is generally not the most characteristic or notable electrolyte abnormality in pyloric stenosis compared to the depletion of chloride and potassium or the development of metabolic alkalosis.

In summary, hypernatremia is the least likely to be observed because the clinical picture of pyloric stenosis predominantly presents with the other electrolyte disturbances arising from chronic vomiting.