A 60-year-old man taking prazosin for benign prostatic hyperplasia develops sudden severe hypotension on standing after his first dose. The mechanism responsible is BEST described as:

• A Reflex bradycardia due to baroreceptor activation
• B Direct negative inotropic effect reducing cardiac output
• C Beta-2 receptor activation causing arteriolar vasodilation
• D Loss of alpha-1–mediated venoconstriction leading to venous pooling and reduced preload ✓

Explanation

First-dose hypotension with prazosin is caused by blockade of alpha-1 adrenoceptors on veins, abolishing the tonic venoconstriction that maintains venous return; on standing, blood pools in the periphery, preload falls, and cardiac output drops sharply. The heart rate may actually rise (not fall) due to baroreflex. There is no direct cardiac depression or beta-2 involvement.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.