## **Core Concept**
Congenital hypertrophic pyloric stenosis (CHPS) is a condition characterized by the thickening of the pylorus muscle in infants, leading to gastric outlet obstruction. This condition results in severe vomiting, which can cause various metabolic disturbances. The primary issue in CHPS is the obstruction of the gastric outlet, leading to loss of gastric contents.

## **Why the Correct Answer is Right**
The correct answer, **metabolic alkalosis**, occurs due to the persistent vomiting of gastric contents, which are rich in hydrochloric acid (HCl). The loss of HCl leads to a decrease in chloride ions (hypochloremia) and hydrogen ions in the blood, causing a rise in blood pH (alkalosis). The body compensates for the loss of hydrogen ions by retaining more bicarbonate ions, further contributing to the alkalosis. This condition is characterized by an increased pH and bicarbonate levels in the blood.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Metabolic acidosis is incorrect because, although it can occur in some cases of vomiting (especially when there is a component of renal failure or severe dehydration), it is not the typical metabolic abnormality seen in CHPS. In CHPS, the vomiting of acid-rich gastric contents tends to lead to alkalosis rather than acidosis.
- **Option B:** Respiratory acidosis is incorrect because it involves abnormalities in CO2 levels, typically seen in conditions affecting lung function or respiratory rate, not directly related to the pathophysiology of CHPS.
- **Option C:** Respiratory alkalosis is incorrect because, while it can occur with hyperventilation, it is not directly related to the metabolic disturbances caused by vomiting in CHPS.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that in congenital hypertrophic pyloric stenosis, the classic laboratory finding is **metabolic alkalosis with hypochloremia and hypokalemia**. The hypokalemia occurs due to the loss of potassium ions in the vomit and the increased excretion of potassium in the urine as a result of secondary hyperaldosteronism.

## **Correct Answer:** . **Metabolic Alkalosis**