A 58-year-old woman with idiopathic overactive bladder develops troublesome dry mouth and constipation on oxybutynin. The drug is switched to mirabegron. What is its mechanism of action?

• A Selective beta-3 adrenergic receptor agonist causing detrusor relaxation ✓
• B Non-selective muscarinic receptor antagonist with calcium channel blocking activity
• C Tricyclic agent inhibiting reuptake of serotonin and noradrenaline
• D Topical estrogen restoring urothelial sensitivity

Explanation

Mirabegron is a selective beta-3 adrenoceptor agonist; stimulation of beta-3 receptors relaxes the detrusor during filling and increases bladder capacity without antimuscarinic side effects, so it is preferred when dry mouth or constipation limits anticholinergic therapy. Oxybutynin itself combines antimuscarinic action with smooth muscle relaxation, imipramine is the tricyclic acting on monoamine reuptake, and topical estrogen is an adjunct in postmenopausal urogenital atrophy rather than a primary overactive bladder drug.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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