**Core Concept**
Acute mountain sickness (AMS) is a condition that occurs when a person ascends to high altitudes too quickly, resulting in a lack of oxygen in the body. The primary mechanism is hypoxia-induced fluid shift from the bloodstream to the lungs, causing pulmonary vasoconstriction and subsequent increased pressure on the brain.

**Why the Correct Answer is Right**
Nifedipine is a calcium channel blocker that relaxes vascular smooth muscle, leading to vasodilation. At high altitudes, vasodilation helps to reduce pulmonary hypertension and alleviate symptoms of AMS. By reducing pulmonary artery pressure, nifedipine improves oxygenation and reduces the risk of cerebral edema.

**Why Each Wrong Option is Incorrect**
**Option A:** Dexamethasone is a corticosteroid used to prevent or treat AMS by reducing inflammation, but it is not the first-line treatment.
**Option B:** Acetazolamide is a carbonic anhydrase inhibitor used to prevent AMS by increasing ventilation and reducing fluid retention, but it is not as effective as nifedipine in acute settings.
**Option C:** Oxygen supplementation is essential in severe AMS, but it is not a specific pharmacological treatment for the condition.

**Clinical Pearl / High-Yield Fact**
Remember the "Lake Louise criteria" for diagnosing AMS: headache, fatigue, dizziness, and nausea or vomiting in a person who has recently ascended to a high altitude.

**Correct Answer:** C. Nifedipine