What symptom is most likely to occur when PO2 levels fall below 60?

When PO2 levels fall below 60 mmHg, one of the primary concerns is the lack of sufficient oxygen in the blood, a condition known as hypoxemia. This deficiency can lead to inadequate oxygen supply to vital organs, particularly the brain. Syncope, or fainting, is a common response to low oxygen levels because the brain is highly sensitive to changes in oxygen concentration. The brain requires a constant supply of oxygen to function properly, and when this supply drops, it can lead to a loss of consciousness.

On the other hand, severe headaches can be related to various issues, including elevated carbon dioxide levels or increased intracranial pressure, but they are not as directly linked to low PO2 levels as syncope. Increased energy levels would be unlikely due to the body's response to oxygen deprivation, which would usually lead to fatigue or lethargy. Fever does not necessarily correlate with low PO2 levels and is typically associated more with infection or inflammation rather than oxygen levels.

Thus, syncope stands out as the most likely symptom when PO2 levels drop below 60 mmHg due to the critical impact of insufficient oxygenation on cerebral function.