Phenoxybenzamine differs from phentolamine in its mechanism of alpha-adrenoceptor blockade primarily because phenoxybenzamine:

• A Is selective for alpha-2 over alpha-1 receptors
• B Is a partial agonist at alpha-1 receptors
• C Blocks both alpha and beta receptors equally
• D Forms a covalent (irreversible) bond with the receptor ✓

Explanation

Phenoxybenzamine is a haloalkylamine that forms a stable covalent bond with alpha receptors, making its blockade irreversible; the effect lasts 24–48 hours until new receptors are synthesized. Phentolamine, in contrast, is a competitive (reversible) alpha-1/alpha-2 blocker with a shorter duration of action. The irreversibility of phenoxybenzamine explains why its effect cannot be overcome by high doses of catecholamines, making it useful in pheochromocytoma pre-operative preparation.

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

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