A patient receives a drug that produces mydriasis without cycloplegia and is used to facilitate ophthalmic examination. Which receptor selectivity best explains this pharmacological profile?
- A Selective muscarinic M3 blockade
- B Selective alpha-1 adrenoceptor agonism ✓
- C Non-selective muscarinic antagonism
- D Beta-2 agonism
Correct answer: B. Selective alpha-1 adrenoceptor agonism
Explanation
Phenylephrine (alpha-1 agonist) causes mydriasis by contracting the iris dilator muscle without affecting the ciliary muscle; accommodation is therefore preserved (no cycloplegia). Non-selective muscarinic antagonists (e.g., atropine, tropicamide) cause both mydriasis AND cycloplegia because they block M3 receptors on both the iris sphincter and ciliary muscle. Selective M3 blockade would also produce cycloplegia.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.