What might a casualty describe if they are experiencing a headache?

When a casualty reports experiencing a headache, this is best understood as a symptom of their condition. Symptoms are subjective experiences that an individual reports, which may indicate an underlying issue or injury. In this context, the headache can suggest various conditions, ranging from mild stress or dehydration to more severe issues like a concussion or intracranial pressure.

Considering the other choices provides additional context: while a headache could be a sign of severe injury, it is not always a definitive indicator on its own without further evaluation. A visible indication of trauma typically refers to observable signs (like bruising or swelling), which would not necessarily be present with just a headache. Finally, while medical personnel might use the report of a headache as part of their diagnostic process, it is primarily the casualty’s subjective experience that classifies it as a symptom. Thus, understanding it as a symptom helps in recognizing the need for further assessment of the casualty's overall condition.