**Core Concept**
Congenital pyloric stenosis is a condition characterized by the narrowing of the pyloric channel, leading to gastric outlet obstruction. This condition is associated with a biochemical abnormality that reflects the body's response to the obstruction.

**Why the Correct Answer is Right**
The most common biochemical abnormality seen in congenital pyloric stenosis is hypochloremic metabolic alkalosis. This occurs due to the loss of hydrochloric acid (HCl) from the stomach, which is a result of the vomiting that accompanies the condition. The loss of HCl leads to a decrease in chloride ions and an increase in bicarbonate ions in the blood, causing metabolic alkalosis.

**Why Each Wrong Option is Incorrect**
* **Option A:** Hyperkalemia is not a common biochemical abnormality seen in congenital pyloric stenosis. In fact, hypokalemia (low potassium levels) is more commonly associated with this condition due to the loss of potassium ions in the vomit.
* **Option B:** Hypokalemia is a possible biochemical abnormality in congenital pyloric stenosis, but it is not the most common. Hypochloremic metabolic alkalosis is the hallmark biochemical abnormality of this condition.
* **Option D:** Hypercalcemia is not typically associated with congenital pyloric stenosis. The biochemical abnormalities seen in this condition are more related to the loss of gastric acid and the subsequent metabolic alkalosis.

**Clinical Pearl / High-Yield Fact**
In cases of congenital pyloric stenosis, the biochemical abnormality of hypochloremic metabolic alkalosis can be treated with nasogastric tube placement to aspirate gastric contents and restore acid-base balance.

**Correct Answer:** C.