Which agent is primarily used for mydriasis during an eye exam?

Phenylephrine is primarily used for mydriasis during eye examinations because it functions as an adrenergic agonist, specifically a selective alpha-1 receptor agonist. When applied to the eye, it causes contraction of the dilator muscle of the iris, leading to pupillary dilation. This effect is achieved without significant impact on the ciliary muscle, meaning accommodation (the eye's ability to focus on near objects) is typically preserved. This characteristic makes phenylephrine particularly suitable for quick and effective dilation needed during ocular assessments.

In contrast, the other agents listed have different primary uses. Apraclonidine is primarily used to reduce intraocular pressure in glaucoma patients and has minimal effect on pupil size. Cyclopentolate, while capable of inducing mydriasis, is more often used to paralyze accommodation for refractive error assessments rather than solely for mydriasis. Brimonidine is also used in managing intraocular pressure and has some mydriatic effect, but it is not primarily indicated for dilation in a clinical setting. Therefore, phenylephrine stands out as the main agent employed for mydriasis during an eye exam due to its targeted mechanism and rapid action.