Phenoxybenzamine is used in pre-operative management of phaeochromocytoma rather than a short-acting alpha-blocker because it:

• A Is selective for alpha-2 receptors, preventing catecholamine release
• B Forms an irreversible covalent bond with alpha receptors, providing prolonged blockade ✓
• C Has no reflex tachycardia owing to concurrent beta-1 blockade
• D Blocks nicotinic receptors at the adrenal medulla

Explanation

Phenoxybenzamine is a non-selective, irreversible (non-equilibrium competitive) alpha blocker that alkylates the alpha-1 and alpha-2 receptors via an aziridinium intermediate, forming a covalent bond. This ensures sustained blockade for 24–48 hours that cannot be overcome by surges of catecholamines released during tumour manipulation, which is the main hazard of phaeochromocytoma surgery. Reversible alpha-blockers like prazosin can be outcompeted by catecholamine surges. Phenoxybenzamine does not block alpha-2 presynaptically to reduce catecholamine release as its primary mechanism, nor does it have intrinsic beta-blocking properties.

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

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Written and medically reviewed by the StethoPrep medical team.