Alpha adrenergic blocking agent used as a vasodilator?

• A. Phentolamine
• B. Yohimbine
• C. Prazosin
• D. Phenoxybenzamine

Answer: (D)

Blocking alpha receptors on vascular smooth muscle prevents norepinephrine from triggering constriction, so blood vessels dilate and systemic vascular resistance falls. Phenoxybenzamine is the classic example because it irreversibly blocks both alpha-1 and alpha-2 receptors, giving a long-lasting vasodilatory effect. This enduring action is why it’s described as a vasodilator in clinical use, such as preoperative management of pheochromocytoma.

Phentolamine can also cause vasodilation but is reversible and shorter-acting, so its effects don’t persist. Prazosin is selective for alpha-1 blockade, which does cause vasodilation but is more limited in duration and not irreversible. Yohimbine blocks alpha-2 receptors, which can increase sympathetic activity rather than promote dilation, so it isn’t used as a vasodilator.