**Core Concept**
Hyperrophic pyloric stenosis is a condition characterized by the thickening of the pyloric muscle, leading to obstruction of the gastric outlet. This results in persistent vomiting, dehydration, and metabolic disturbances.

**Why the Correct Answer is Right**
In hyperrophic pyloric stenosis, the persistent vomiting leads to a loss of hydrochloric acid (HCl) and gastric contents, which are rich in potassium (K+). This results in hypokalemia (low potassium levels). Additionally, the vomiting also leads to metabolic alkalosis, as the loss of HCl increases the bicarbonate (HCO3-) levels in the blood. The combination of hypokalemia and metabolic alkalosis is a characteristic metabolic disturbance seen in this condition.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because hyperrophic pyloric stenosis does not typically cause hypocalcemia (low calcium levels).
**Option B:** This option is incorrect because while metabolic alkalosis is a feature of hyperrophic pyloric stenosis, it is not the primary metabolic disturbance.
**Option C:** This option is incorrect because hyperrophic pyloric stenosis does not typically cause hypomagnesemia (low magnesium levels).

**Clinical Pearl / High-Yield Fact**
In cases of hyperrophic pyloric stenosis, the metabolic alkalosis is often associated with a compensatory hypochloremic metabolic alkalosis, characterized by low chloride levels and high bicarbonate levels.

**Correct Answer: D. Metabolic alkalosis with hypokalemia.**