Which agent provides effective alpha blockade in preparation for pheochromocytoma surgery?

Phenoxybenzamine is the correct agent for providing effective alpha blockade in preparation for pheochromocytoma surgery. This irreversible and non-selective alpha-adrenergic antagonist effectively prevents the actions of catecholamines, which are typically elevated in patients with pheochromocytoma. By blocking these receptors, phenoxybenzamine helps reduce blood pressure and decrease the risk of hypertensive crises during surgery, making the patient's condition more stable and manageable.

The nature of pheochromocytoma—a tumor that secretes catecholamines like epinephrine and norepinephrine—means that adequate blockade is essential prior to surgical intervention. Phenoxybenzamine's long-lasting effects ensure that the adrenergic receptors remain blocked, assisting in the overall management of blood pressure and minimizing complications related to the tumor's catecholamine release during the perioperative period.

Other agents mentioned, such as nitroglycerin, labetolol, and atropine, may have roles in the management of blood pressure or heart rate under specific conditions, but they do not provide the targeted alpha blockade necessary for preparation for pheochromocytoma surgery.