**Core Concept**
Pheochromocytomas are catecholamine-secreting tumors that can lead to hypertension, tachycardia, and other cardiovascular complications. Managing these patients requires careful consideration of the pharmacological responses to various medications.

**Why the Correct Answer is Right**
Phentolamine is a non-selective α-adrenergic receptor antagonist that can exacerbate the symptoms of pheochromocytoma by blocking the α-adrenergic receptors, which normally help to terminate the effects of circulating catecholamines. This can lead to unopposed β-adrenergic stimulation, resulting in increased heart rate, blood pressure, and cardiac workload. In contrast, other medications listed here can be used cautiously in the management of pheochromocytoma.

**Why Each Wrong Option is Incorrect**
**Option B:** Enalapril is an angiotensin-converting enzyme (ACE) inhibitor that can be used to manage hypertension associated with pheochromocytoma by reducing peripheral resistance and lowering blood pressure.
**Option C:** Clonidine is an alpha-2 adrenergic agonist that can be used to manage hypertension and reduce sympathetic tone in patients with pheochromocytoma.
**Option D:** Methyldopa is a centrally acting antihypertensive agent that can be used to manage hypertension associated with pheochromocytoma without exacerbating the symptoms.

**Clinical Pearl / High-Yield Fact**
When managing patients with pheochromocytoma, it is essential to use medications that do not worsen the symptoms of catecholamine excess. Phentolamine should be avoided as a first-line treatment due to its potential to exacerbate hypertension and tachycardia.

**✓ Correct Answer: A. Phentolamine**