**Core Concept**
Subarachnoid hemorrhage is a life-threatening condition characterized by bleeding into the space surrounding the brain, often due to the rupture of an aneurysm or arteriovenous malformation. The primary goal in managing subarachnoid hemorrhage is to prevent further bleeding and reduce the risk of rebleeding, while also addressing potential complications such as vasospasm and cerebral edema.

**Why the Correct Answer is Right**
Nimodipine is a calcium channel blocker that selectively inhibits L-type calcium channels in vascular smooth muscle cells, which are overexpressed in the cerebral vasculature following subarachnoid hemorrhage. By blocking these channels, nimodipine reduces the contraction of cerebral arteries and veins, thereby decreasing the risk of vasospasm and improving blood flow to the affected brain tissue. This mechanism of action is particularly beneficial in the acute phase of subarachnoid hemorrhage, where vasospasm is a major concern.

**Why Each Wrong Option is Incorrect**
**Option A:** Amlodipine is a non-selective calcium channel blocker that primarily affects the heart and peripheral vasculature, making it less suitable for the treatment of subarachnoid hemorrhage.

**Option B:** Diltiazem is a non-selective calcium channel blocker that can cause significant vasodilation, which may exacerbate cerebral edema in the setting of subarachnoid hemorrhage.

**Option C:** Verapamil is a non-selective calcium channel blocker that can cause significant vasodilation and is not typically used to prevent vasospasm in the context of subarachnoid hemorrhage.

**Clinical Pearl / High-Yield Fact**
Nimodipine is specifically indicated for the prevention of cerebral vasospasm following subarachnoid hemorrhage, and its use should be initiated as soon as possible after the hemorrhage.

**✓ Correct Answer: A. Nimodipine**