Acute Mountain Sickness (AMS): This is the most likely diagnosis given the context. AMS occurs when the body cannot adapt well to high altitudes, leading to symptoms such as headache, nausea, fatigue, and shortness of breath. The age of 30 does not preclude this diagnosis, as AMS can affect anyone regardless of age if they ascend too quickly to high altitudes.

Viral or Bacterial Infections: Infections such as the common cold, flu, or bacterial infections like pneumonia could present with similar symptoms to AMS, especially if the individual has recently traveled or been exposed to others who are sick.
Dehydration: Not drinking enough water, especially in high-altitude environments where dehydration can occur more quickly due to increased urine production, can lead to symptoms that mimic AMS, including headache and fatigue.
Altitude-Exacerbated Chronic Conditions: Conditions like migraines or chronic obstructive pulmonary disease (COPD) can be worsened by high altitude, presenting with symptoms that could be confused with AMS.

High-Altitude Pulmonary Edema (HAPE): A potentially fatal condition that can occur at high altitudes, characterized by fluid buildup in the lungs. It's crucial to distinguish HAPE from AMS, as it requires immediate descent and potentially medical intervention.
High-Altitude Cerebral Edema (HACE): Another life-threatening condition where fluid builds up in the brain at high altitudes. Symptoms can include severe headache, confusion, and loss of coordination, which must be recognized and treated promptly.
Carbon Monoxide Poisoning: If the individual has been using fuel-burning appliances in an enclosed space at high altitude, carbon monoxide poisoning could be a deadly diagnosis that needs to be considered, especially if there are symptoms like headache, dizziness, and confusion.

Subarachnoid Hemorrhage: Although rare, a subarachnoid hemorrhage (bleeding into the space surrounding the brain) could present with a sudden, severe headache and might be considered in the differential diagnosis, especially if other symptoms like stiff neck or vomiting are present.
Other Rare Altitude-Related Conditions: Conditions such as altitude-induced thrombosis or other rare cardiovascular events could theoretically occur but are much less common and would typically be considered after ruling out more likely diagnoses.