A patient with pheochromocytoma is prepared for surgery. Alpha-blockade is initiated with phenoxybenzamine. Which of the following best describes its pharmacological mechanism?

• A Competitive, reversible blockade of alpha-1 adrenoceptors only
• B Selective, reversible blockade of alpha-2 adrenoceptors
• C Competitive, reversible blockade of alpha-1 and alpha-2 adrenoceptors
• D Non-competitive, irreversible blockade of both alpha-1 and alpha-2 adrenoceptors ✓

Explanation

Phenoxybenzamine forms a covalent bond with alpha adrenoceptors, resulting in non-competitive irreversible blockade of both alpha-1 and alpha-2 subtypes; this is its key distinguishing feature from competitive alpha-blockers like phentolamine. The irreversible nature ensures sustained blockade despite catecholamine surges during tumour manipulation. Recovery requires synthesis of new receptors, making its duration of action independent of plasma half-life.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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