**Core Concept**
Metabolic alkalosis is a condition characterized by an increase in blood pH due to an excess of bicarbonate ions (HCO3-) or a decrease in chloride ions (Cl-) in the blood. This occurs when the body loses too much acid or gains too much base, leading to an imbalance in the acid-base equilibrium.

**Why the Correct Answer is Right**
The commonest cause of metabolic alkalosis is often related to the gastrointestinal system, particularly in cases of vomiting. When a person vomits, they lose hydrochloric acid (HCl) from the stomach, leading to a decrease in chloride ions and an increase in bicarbonate ions. This shift in ion balance causes the blood pH to rise, resulting in metabolic alkalosis. The kidneys also play a role in correcting this imbalance by reabsorbing more sodium and water, which helps to retain bicarbonate ions in the body.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while certain medications can cause metabolic alkalosis, they are not the most common cause.
**Option B:** This option is incorrect because while certain respiratory conditions can lead to metabolic alkalosis, they are not the most common cause.
**Option C:** This option is incorrect because while certain renal conditions can lead to metabolic alkalosis, they are not the most common cause.

**Clinical Pearl / High-Yield Fact**
A classic mnemonic to remember the causes of metabolic alkalosis is "VOMIT": V - Vomiting (most common cause), O - Overuse of nasogastric suction, M - Mineralocorticoid excess, I - Ingestion of alkaline substances, T - Thyroid hormone excess.

**Correct Answer:** B. Vomiting.