Which is a selective alpha-1 adrenergic agonist used for mydriasis?

• A. Apraclonidine
• B. Brimonidine
• C. Phenylephrine
• D. Dipivefrin

Answer: (C)

Dilation of the pupil is accomplished by stimulating the iris dilator muscle via alpha-1 receptors, which causes the radial muscle to contract and pull the pupil open. Phenylephrine is a direct-acting alpha-1 adrenergic agonist, so it reliably produces mydriasis when applied as an eye drop and largely preserves accommodation.

Apraclonidine and brimonidine are mainly alpha-2 agonists used to lower intraocular pressure; they don’t target the iris dilator as effectively, so their mydriatic effect is less reliable. Dipivefrin is a prodrug of epinephrine with mixed alpha and beta activity, not selective for alpha-1, and it’s used more for lowering IOP than for dilation. Thus, phenylephrine best fits the description of a selective alpha-1 agonist used for mydriasis.