**Core Concept**
Cerebral vasospasm is a complication of subarachnoid hemorrhage, leading to reduced blood flow and potential infarction. The goal of treatment is to reverse vasospasm and restore cerebral blood flow.

**Why the Correct Answer is Right**
The correct answer, **Methylene Blue**, is used to reverse cerebral vasospasm by inhibiting the activity of nitric oxide synthase (NOS), which is overexpressed in the cerebral arteries following subarachnoid hemorrhage. This inhibition leads to decreased production of nitric oxide (NO), a potent vasodilator, resulting in relaxation of the smooth muscle cells and reversal of vasospasm. Methylene Blue has been shown to be effective in clinical trials for this indication.

**Why Each Wrong Option is Incorrect**
**Option A:** **Nicardipine** is a calcium channel blocker used to treat hypertension and cerebral vasospasm, but it is not the primary treatment for reversal of cerebral vasospasm following subarachnoid hemorrhage.
**Option B:** **Nimodipine** is a calcium channel blocker used to prevent cerebral vasospasm, but it does not reverse existing vasospasm.
**Option C:** **Fenoldopam** is a dopamine receptor agonist used to treat hypertension, but it is not used for reversal of cerebral vasospasm.

**Clinical Pearl / High-Yield Fact**
Methylene Blue is a vasodilator that can be used to reverse cerebral vasospasm following subarachnoid hemorrhage, but it requires careful dosing and monitoring due to potential side effects.

**Correct Answer:** D. Methylene Blue