In a patient with sudden severe headache and rapid decline, the most likely diagnosis is?

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This presentation is classic for subarachnoid hemorrhage due to rupture of a intracranial aneurysm. A sudden, severe “thunderclap” headache with rapid neurological decline points to an acute bleed into the subarachnoid space, rather than a migrainous attack or a slowly growing mass. In SAH, patients can deteriorate quickly as blood increases intracranial pressure and can trigger hydrocephalus or cerebral vasospasm.

In practice, the first step is emergent imaging with a noncontrast head CT because it rapidly detects most SAH within the first hours of onset. If the CT is negative but clinical suspicion remains high, proceed with lumbar puncture to look for xanthochromia or elevated red blood cells in the CSF. Once SAH is diagnosed, urgent neurosurgical consultation is essential to secure the aneurysm (surgical clipping or endovascular coil). Blood pressure should be carefully controlled to minimize risk of rebleeding, and nimodipine is used to reduce the risk of delayed cerebral ischemia from vasospasm. Recognize that other causes like migraine or brain tumor do not typically present with this abrupt, maximal headache and rapid decline.