Final answer:
The body system that would be necessary to examine during the secondary assessment of the patient is the Neurological system.
Step-by-step explanation:
The body system that would be necessary to examine during the secondary assessment of the 25-year-old patient is the Neurological system. After being struck in the head and experiencing a brief moment of unresponsiveness, the patient's complaint of a headache indicates potential neurological involvement. Assessing the neurological system would involve checking reflexes, sensory responses, and motor control to determine any deficits or abnormalities.