Naphazoline is an alpha-adrenergic agonist used in eye drops. Which statement best describes its action?

• A. Constricts conjunctival vessels via alpha-adrenergic agonism
• B. Dilates pupil by stimulating the iris dilator
• C. Increases aqueous humor production
• D. Blocks acetylcholine receptors

Answer: (A)

Naphazoline works by stimulating alpha-adrenergic receptors on the small blood vessels of the conjunctiva. This direct receptor activation causes the vascular smooth muscle to contract, leading to constriction of the superficial conjunctival vessels. With those vessels narrowed, red and swollen appearance lessens, which is why this drug is used as an ocular decongestant.

Pupil dilation would involve stimulation of the iris dilator muscle, which naphazoline does not primarily do, and increasing aqueous humor production would require different mechanisms not associated with this vasoconstrictor. Blocking acetylcholine receptors would be an anticholinergic action and isn’t how naphazoline works. So the essential effect is the vasoconstriction of conjunctival vessels via alpha-adrenergic agonism.