What is the next best line of action after a patient with pyloric stenosis has been vomiting profusely?

After a patient with pyloric stenosis has been vomiting profusely, the most appropriate next step is to check serum potassium levels. This is crucial because repeated vomiting can lead to significant electrolyte imbalances, particularly hypokalemia (low potassium levels), which can occur due to the loss of gastric acid and fluid.

Monitoring serum electrolytes is vital before any surgical intervention, as electrolyte abnormalities can increase the risk of complications during anesthesia and surgery. Therefore, ensuring that the patient has normal potassium levels helps to stabilize their condition.

While fluid replacement and surgery are important components of patient management, they should be approached only after confirming the patient's electrolyte status. In this case, checking serum potassium before administering any fluids or proceeding to surgery is the safest and most effective approach.