Final answer:
The initial test in this case should be a CT scan.
Step-by-step explanation:
The initial test in this case should be a CT scan. The patient presented with sudden headache, collapsing, and some lethargy. The rigid neck, absence of papilledema, focal cranial nerve or motor signs suggest a potential neurological issue. A CT scan can help diagnose any possible structural abnormalities or bleeding in the brain, such as subarachnoid hemorrhage or meningitis. This is a crucial step in determining the cause of the symptoms and guiding further treatment.