A 70-year-old man with benign prostatic hyperplasia is given an antimuscarinic drug for overactive bladder. Two days later he presents with a distended, palpable bladder and inability to void. Which property of the antimuscarinic drug best explains this adverse effect?

• A Inhibition of acetylcholine release from pelvic parasympathetic ganglia
• B Stimulation of alpha-1 receptors at the bladder neck increasing outlet resistance
• C Blockade of nicotinic receptors at the external urethral sphincter
• D Blockade of M3 receptors on detrusor smooth muscle preventing contraction ✓

Explanation

Voiding requires parasympathetic M3-mediated contraction of the detrusor muscle. Antimuscarinics such as oxybutynin block this contraction, and in a man whose outflow is already compromised by prostatic enlargement, acute urinary retention results. This is a classic warning: anticholinergic drugs are relatively contraindicated in BPH. The sphincter and ganglia are not the site of action, and antimuscarinics do not stimulate alpha-1 receptors.

Reference: Katzung's Basic and Clinical Pharmacology, 15th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.