**Core Concept**
Metabolic alkalosis is a condition characterized by an elevated blood pH and bicarbonate level, often resulting from an imbalance in acid-base homeostasis. In this scenario, the patient's bicarbonate level is 34 mEq/L, indicating a significant alkalotic state.

**Why the Correct Answer is Right**
The most common cause of metabolic alkalosis in this age group is excessive vomiting, leading to loss of hydrochloric acid (HCl) and a subsequent decrease in chloride levels. This results in a relative increase in hydrogen ions (H+) being reabsorbed in the kidneys, leading to an increase in bicarbonate reabsorption and subsequently, an increase in blood bicarbonate levels. The kidneys compensate for the alkalosis by excreting more bicarbonate, but in this case, the body's buffering capacity is overwhelmed, resulting in the observed metabolic alkalosis.

**Why Each Wrong Option is Incorrect**
**Option A:** This choice is incorrect because while it is a possible cause of metabolic alkalosis, it is less common than excessive vomiting in this age group.
**Option B:** This option is incorrect because while it can lead to metabolic alkalosis, the patient's bicarbonate level would likely be lower due to the compensatory mechanisms of the kidneys.
**Option C:** This choice is incorrect because while it can cause metabolic alkalosis, the primary mechanism involves the loss of potassium ions, not the loss of hydrogen ions.

**Clinical Pearl / High-Yield Fact**
In patients with metabolic alkalosis, it's essential to assess for potential causes of chloride depletion, such as excessive vomiting or nasogastric suction. This can help guide treatment and prevent complications.

**Correct Answer: C. Excessive vomiting**