**Core Concept**
Hypertrophic pyloric stenosis (HPS) is a condition characterized by the thickening of the pyloric muscle, leading to gastric outlet obstruction. The metabolic disturbance in HPS is primarily due to prolonged vomiting, which results in loss of hydrochloric acid (HCl) and subsequent metabolic alkalosis.

**Why the Correct Answer is Right**
The loss of HCl leads to a decrease in hydrogen ions (H+) in the blood, causing an increase in serum bicarbonate levels. This results in metabolic alkalosis, which is characterized by an elevated pH and a decreased partial pressure of carbon dioxide (pCO2). The body tries to compensate for this alkalosis by increasing the respiratory rate, leading to hyperventilation and a decrease in pCO2.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not accurately describe the metabolic disturbance in HPS. While dehydration is a common complication of prolonged vomiting, it is not the primary metabolic disturbance.
* **Option B:** This option is incorrect because it refers to a metabolic acidosis, which is the opposite of the metabolic alkalosis seen in HPS.
* **Option C:** This option is incorrect because it does not accurately describe the metabolic disturbance in HPS. While electrolyte imbalances can occur in HPS, they are not the primary metabolic disturbance.

**Clinical Pearl / High-Yield Fact**
In cases of HPS, it's essential to remember that the metabolic disturbance is primarily due to metabolic alkalosis, which can be confirmed by laboratory tests. Additionally, the presence of hypokalemia (low potassium levels) is a common finding in HPS due to the loss of potassium ions in the vomit.

**Correct Answer:** B. Metabolic alkalosis.