Which of the following correctly describes the mechanism by which prazosin causes first-dose syncope?

• A Central α2 stimulation reducing sympathetic outflow abruptly
• B Blockade of α1 receptors preventing vasoconstriction on standing, causing orthostatic hypotension ✓
• C Inhibition of renin release from juxtaglomerular cells
• D Non-selective β-blockade reducing cardiac output

Explanation

Prazosin selectively blocks postsynaptic α1 adrenergic receptors, preventing arteriolar and venous vasoconstriction. On first dose, this blunts the reflex vasoconstriction needed on standing, producing a sharp orthostatic blood pressure drop and syncope. Clonidine (not prazosin) acts centrally via α2, and prazosin has no β-blocking activity.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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