**Core Concept**
Pheochromocytoma is a rare, catecholamine-secreting tumor that can lead to episodes of severe hypertension. Certain medications can exacerbate this condition by triggering a hypertensive crisis.

**Why the Correct Answer is Right**
The correct answer is **C. Vancomycin**. Vancomycin is an antibiotic that can cause a release of catecholamines from pheochromocytoma cells, leading to a hypertensive crisis. This is because vancomycin can stimulate the release of catecholamines from the tumor cells through an unknown mechanism. The resulting surge in catecholamines can cause severe hypertension, which can be life-threatening.

**Why Each Wrong Option is Incorrect**
* **Option A:** Chloramphenicol is not typically associated with hypertensive crises in pheochromocytoma patients.
* **Option B:** Furosemide is a diuretic that can actually be used to manage hypertension in pheochromocytoma patients, although it should be used with caution.

**Clinical Pearl / High-Yield Fact**
It's essential to note that patients with pheochromocytoma should be managed in a hospital setting, and any new medications should be started with caution and under close monitoring.

**Correct Answer:** C. Vancomycin.