A 68-year-old man develops urinary hesitancy and blurred vision after starting a drug. His prescription includes tamsulosin for BPH and the newly added drug. Which pharmacological property best explains why the new drug causes these anticholinergic side effects while tamsulosin does not?
- A Tamsulosin selectively blocks alpha-1A adrenoceptors expressed in the prostate without muscarinic receptor affinity ✓
- B Tamsulosin activates beta-3 adrenoceptors in the detrusor muscle
- C Tamsulosin inhibits phosphodiesterase-5 to relax smooth muscle
- D Tamsulosin inhibits norepinephrine reuptake transporter (NET)
Correct answer: A. Tamsulosin selectively blocks alpha-1A adrenoceptors expressed in the prostate without muscarinic receptor affinity
Explanation
Tamsulosin is a uroselective alpha-1A/D adrenoceptor antagonist that relaxes prostatic smooth muscle without affecting muscarinic receptors; it therefore does not cause urinary hesitancy, dry mouth, or blurred vision. The newly added drug must possess muscarinic M1/M2/M3 antagonism to produce anticholinergic symptoms. Beta-3 agonism (mirabegron) relaxes detrusor but this is not tamsulosin's mechanism. PDE-5 inhibition and NET blockade are unrelated to tamsulosin's action.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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