What symptom indicates acute mountain sickness?

Acute mountain sickness (AMS) is characterized by a set of symptoms that occur when individuals ascend to high altitudes too quickly without proper acclimatization. One of the most prevalent symptoms of AMS is headaches, which often arise as a response to lower oxygen levels and increased altitude. These headaches can vary in intensity and may be accompanied by other symptoms such as nausea, dizziness, or fatigue.

While symptoms like rapid heartbeat, increased appetite, and chronic fatigue may occur in various conditions, they are not specifically indicative of acute mountain sickness. For instance, a rapid heartbeat can result from several factors, including anxiety or exertion, and is not unique to AMS. Increased appetite often doesn't relate to the physiological changes caused by altitude sickness; instead, many individuals experience a decreased appetite at high altitudes. Chronic fatigue can reflect various health issues or lifestyle factors but is not a primary symptom of AMS. Thus, headaches stand out as a key and defining symptom of acute mountain sickness, making it the most appropriate choice in this context.