A patient with angle-closure glaucoma is prescribed a drug for urinary incontinence. Which muscarinic receptor subtype-selective drug would be SAFEST with least concern for raising intraocular pressure?

• A Oxybutynin (non-selective M1/M2/M3 blocker)
• B Darifenacin (selective M3 blocker with low CNS penetration) ✓
• C Tolterodine (non-selective with moderate M3 affinity)
• D Fesoterodine (pro-drug of 5-hydroxymethyl tolterodine)

Explanation

Darifenacin is selective for M3 muscarinic receptors and has low CNS penetration due to its P-glycoprotein substrate nature. While all muscarinic blockers risk precipitating angle-closure glaucoma, darifenacin's M3 selectivity combined with peripheral tissue preference makes it relatively safer than non-selective agents. M3 blockade in the ciliary muscle reduces aqueous production but cycloplegia risk is less systemic with darifenacin versus oxybutynin which crosses the blood-brain barrier readily.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.