Final answer:
Nimodipine is the medication used to prevent vasospasm following a subarachnoid hemorrhage, as it is a calcium channel blocker that can reduce the constriction of blood vessels in the brain.
Step-by-step explanation:
The agent given to prevent vasospasm after subarachnoid hemorrhage is a) Nimodipine. Nimodipine is a calcium channel blocker that is used specifically to prevent cerebral vasospasm, which is a potential complication following subarachnoid hemorrhage. This condition arises when blood from a ruptured aneurysm irritates the blood vessels, causing them to constrict and potentially leading to ischemic stroke.
Agents like mannitol and furosemide are diuretics, and while they may help to reduce intracranial pressure, they do not directly prevent vasospasm. Phenytoin is an antiepileptic drug and does not have a role in the prevention of vasospasm. Therefore the correct answer is Nimodipine which has been shown to be effective in reducing the risk of ischemic deficits after a subarachnoid hemorrhage due to its vasodilatory effects on cerebral vessels.