A patient with pheochromocytoma crisis develops severe hypertension and tachycardia after a single dose of propranolol administered without prior alpha-blockade. The worsening hypertension is best explained by:

• A Beta-2 mediated renin release amplifying angiotensin II
• B Unopposed alpha-1 adrenoceptor stimulation by circulating catecholamines ✓
• C Rebound norepinephrine surge from presynaptic beta-2 blockade
• D Central sympathetic activation via beta-1 blockade in medulla

Explanation

In pheochromocytoma, catecholamines stimulate both alpha and beta receptors. Beta-2 adrenoceptors mediate vasodilation in skeletal muscle vasculature; when propranolol blocks beta-2 without prior alpha-blockade, the vasodilatory counter-balance is removed while alpha-1 stimulation by circulating epinephrine/norepinephrine remains unopposed, causing paradoxical severe hypertension. This is why phenoxybenzamine or phentolamine must precede beta-blockade. The other options describe real pharmacological effects but are not the primary mechanism of this paradox.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.