**Core Concept**
Hyperrophic pyloric stenosis (HPS) is a condition characterized by the thickening of the pyloric muscle, leading to gastric outlet obstruction. This results in severe vomiting, dehydration, and electrolyte imbalances. In HPS, the loss of gastric contents through vomiting leads to a specific pattern of electrolyte disturbances.

**Why the Correct Answer is Right**
The correct answer is Chloride because vomiting leads to the loss of gastric contents, which are rich in hydrochloric acid (HCl). HCl is composed of hydrogen ions (H+) and chloride ions (Cl-). As a result, the loss of HCl through vomiting causes a significant decrease in chloride levels. Chloride is a crucial electrolyte that helps maintain fluid balance and pH levels in the body. Replenishing chloride is essential in the management of HPS to correct the electrolyte imbalance and prevent complications.

**Why Each Wrong Option is Incorrect**

* **Option A:** Sodium is an important electrolyte, but it is not the primary electrolyte lost in HPS. While sodium levels may be affected due to dehydration and vomiting, chloride replenishment is more critical in this condition.
* **Option B:** Hydrogen ions (H+) are indeed lost in HPS due to the vomiting of gastric contents. However, the question asks for the electrolyte that should be replenished, not the ion that is lost. Chloride is the electrolyte that needs to be replenished to correct the imbalance.
* **Option D:** Bicarbonate is not directly related to the electrolyte imbalance in HPS. While bicarbonate levels may be affected in metabolic disturbances, it is not the primary electrolyte that needs to be replenished in this condition.

**Clinical Pearl / High-Yield Fact**
In HPS, it is essential to remember that the loss of gastric contents leads to a chloride deficit. Chloride replenishment is critical to correct the electrolyte imbalance and prevent complications such as seizures, tetany, and cardiac arrhythmias. A mnemonic to remember is "C-H-L-O-R-I-D-E" - Correcting the Chloride deficit is the key to managing HPS.

**✓ Correct Answer: C. Chloride**