The mydriasis produced by tropicamide during a routine ophthalmic examination is primarily due to blockade of which receptor on the iris sphincter pupillae?
- A M3 muscarinic receptor ✓
- B M1 muscarinic receptor
- C Alpha-1 adrenoceptor
- D Nicotinic receptor
Correct answer: A. M3 muscarinic receptor
Explanation
The iris sphincter pupillae is innervated by parasympathetic fibres and contracts via M3 muscarinic receptor activation. Tropicamide, a short-acting antimuscarinic, blocks M3 receptors on the sphincter, causing mydriasis and cycloplegia. M1 receptors are predominantly CNS and gastric, and alpha-1 adrenoceptors mediate dilation via the dilator pupillae—a different mechanism used by phenylephrine drops.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.