A 72-year-old man started on oxybutynin for overactive bladder presents two days later with acute urinary retention and a palpable bladder. The drug produced retention by blocking which receptor on which target?

• A Alpha-1 receptors on the internal urethral sphincter
• B Beta-3 receptors on the detrusor muscle
• C Nicotinic receptors on the external urethral sphincter
• D Muscarinic M3 receptors on the detrusor muscle ✓

Explanation

Voiding depends on parasympathetic pelvic nerve activation of M3 receptors contracting the detrusor. Oxybutynin blocks these muscarinic receptors, relaxing the detrusor and preventing emptying, hence retention. Option A describes the receptor whose stimulation closes the internal sphincter during storage; blocking it would favour leakage, not retention. Beta-3 receptors promote detrusor relaxation during filling, and the external sphincter is somatic (pudendal), not nicotinic-autonomic.

Reference: Katzung's Basic and Clinical Pharmacology, 16th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.