Certified Paraoptometric (CPO) Practice Exam 2025

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Which of the following substances is considered an eye dilator?

Tropicamide
Proparacaine
Lidocaine
Phenylephrine

The correct answer is: Phenylephrine

Phenylephrine is considered an eye dilator because it is an alpha-1 adrenergic agonist that causes dilation of the pupil, a process known as mydriasis. It achieves this effect by stimulating the dilator muscle of the iris, leading to widening of the pupil, which is beneficial during fundoscopic examinations or certain eye procedures to improve visibility of the retina and other structures. This mydriatic effect makes phenylephrine a common choice in ophthalmic practice. In contrast, other substances listed do not promote pupil dilation in the same manner. Tropicamide, while it is a cycloplegic agent that can also dilate the pupil, primarily works by paralysis of the ciliary muscle for refractive assessment. Proparacaine and lidocaine are anesthetics and are not used for dilation; rather, they are employed to numb the eye for minor procedures. Thus, phenylephrine is specifically recognized for its function as a pupil dilator.