**Core Concept**
Hyperrophic pyloric stenosis (HPS) is a condition characterized by the thickening of the pyloric muscle, leading to gastric outlet obstruction. This obstruction results in a characteristic metabolic disturbance due to prolonged gastric emptying and subsequent malabsorption.

**Why the Correct Answer is Right**
The metabolic disturbance in HPS is primarily due to **hypochloremic metabolic alkalosis**. This occurs because the vomiting of gastric contents (which are acidic) leads to a loss of hydrogen ions (H+) and chloride ions (Cl-). As a result, the body retains more bicarbonate ions (HCO3-) to compensate for the lost H+, causing an increase in blood pH and a decrease in chloride levels. The kidneys try to conserve sodium and water by increasing sodium reabsorption and water reabsorption, but this leads to increased potassium excretion, causing hypokalemia.

**Why Each Wrong Option is Incorrect**
**Option A:** Hyperkalemia - This is incorrect because the kidneys excrete potassium ions in response to the increased sodium reabsorption and water reabsorption, leading to hypokalemia.

**Option B:** Metabolic acidosis - This is incorrect because the loss of hydrogen ions and the retention of bicarbonate ions lead to an increase in blood pH, resulting in metabolic alkalosis.

**Option C:** Hyponatremia - This is incorrect because the kidneys try to conserve sodium and water by increasing sodium reabsorption, leading to hypernatremia.

**Clinical Pearl / High-Yield Fact**
In cases of HPS, the metabolic alkalosis can be treated with nasogastric suction to remove the acidic gastric contents and with potassium supplements to correct the hypokalemia.

**Correct Answer: B. Metabolic alkalosis**