What symptom is considered a red flag for a serious head injury in a young adult?

Contralateral extremity weakness is indeed a significant indicator of a serious head injury in a young adult. This symptom suggests that there may be involvement of the brain's motor pathways, specifically those that control movement on one side of the body. Such weakness can indicate a more severe underlying condition, possibly due to increased intracranial pressure, a cerebral contusion, or even a diffuse axonal injury.

In cases of head trauma, one of the critical assessments is to evaluate neurological function, including motor strength. Weakness in the opposite arm or leg compared to the side of the injury can indicate that the injury has affected the areas of the brain responsible for motor control, which typically reside in the opposite hemisphere. This makes contralateral extremity weakness a particularly concerning finding.

Other symptoms like amnesia, loss of consciousness, or a delayed headache can also indicate a serious injury, but they may not be as immediate or direct an indicator of localized brain injury or potential deterioration as contralateral extremity weakness. Immediate medical evaluation is crucial when this symptom is present, as it can inform the need for interventions such as imaging studies or surgical evaluation to prevent further complications.