**Core Concept**
Acute mountain sickness (AMS) is a condition that occurs when the body cannot adapt to high altitudes, leading to inadequate oxygenation of tissues. The primary mechanism involves impaired gas exchange due to hypoxia, which triggers a cascade of physiological responses.

**Why the Correct Answer is Right**
The correct answer is **Nifedipine**. Nifedipine is a calcium channel blocker that works by relaxing vascular smooth muscle, thereby increasing blood flow to the lungs and improving oxygen delivery to the body. This action helps mitigate the effects of hypoxia at high altitudes, reducing the risk of AMS. Nifedipine's vasodilatory effects also decrease pulmonary artery pressure, which can help alleviate symptoms of AMS.

**Why Each Wrong Option is Incorrect**
**Option A:** **Acetazolamide** is used to treat AMS, but it works differently. It inhibits carbonic anhydrase, leading to increased excretion of bicarbonate and a mild metabolic acidosis, which helps stimulate breathing and improve oxygenation. This is not the same mechanism as nifedipine.
**Option B:** **Furosemide** is a diuretic that can be used to treat high-altitude pulmonary edema (HAPE), a complication of AMS, but it is not used to prevent AMS.
**Option C:** **Doxycycline** is an antibiotic that has been used to prevent malaria, but it has no role in preventing AMS.

**Clinical Pearl / High-Yield Fact**
To remember the key mechanism of nifedipine in preventing AMS, recall that calcium channel blockers like nifedipine "open up" blood vessels, allowing more oxygen to reach the body's tissues.

**Correct Answer:** C. Nifedipine.