Repeated USMLE Questions Step 1- 177

A 27-year-old woman, who was diagnosed recently with pheochromocytoma, presents to the ER with high blood pressure. Treatment with phentolamine is initiated. Heart rate monitoring shows tachycardia after administration of her medicine.

What is the mechanism of action of phentolamine in this case?

A-Blockade of cholinergic receptors

B-Blockade of alpha-adrenergic receptors

C-Blockade of beta-adrenergic receptors

D-Direct vasoconstriction of blood vessels

E- Stimulation of adrenal gland to release epinephrine

Answer







Phentolamine is a non-selective alpha adrenergic receptor blocker. Its primary action is blocking of alpha1 receptors causing vasodilation. Because this drug is non-selective alpha blocker so, it also inhibits alpha2 receptors, which normally acts as negative feedback for epinephrine release, leading to persistent tachycardia.

The correct answer is B