Which drug is a direct-acting cholinergic agonist used for pupillary constriction?

• A. Edrophonium
• B. Donazepil
• C. Atropine
• D. Pilocarpine

Answer: (D)

Pupillary constriction is achieved by directly stimulating the muscarinic receptors on the sphincter pupillae of the iris. Pilocarpine is a direct-acting muscarinic agonist, so it binds these receptors and causes the sphincter to contract, producing miosis. It’s used topically in ophthalmology to induce pupil constriction and to help increase aqueous humor outflow, lowering intraocular pressure in glaucoma.

The other drugs work differently: edrophonium is an acetylcholinesterase inhibitor, increasing acetylcholine levels indirectly rather than directly activating receptors; donepezil is another acetylcholinesterase inhibitor with systemic uses; atropine is a muscarinic antagonist that blocks receptors and causes pupil dilation, not constriction.