What is the hallmark symptom of acute mountain sickness?

• A. Dizziness
• B. Fluid retention
• C. Headache
• D. Nausea

Answer: (B)

At altitude, the body’s response to reduced oxygen often presents with a headache as the most characteristic sign. This headache arises because cerebral hypoxia triggers vasodilation and mild edema in the brain, producing pain that is commonly the earliest and most consistent symptom of acute mountain sickness. Other symptoms like dizziness, nausea, or fatigue can accompany it, but the headache is the defining feature clinicians look for. Fluid retention does not typify acute mountain sickness and isn’t the hallmark of this condition; edema-related concerns may appear only in more severe altitude illnesses such as high-altitude cerebral edema, which has its own distinct features. So the best answer is the headache.