Certified Ophthalmic Assistant Practice Exam

Which iris muscle is stimulated by drugs that dilate the pupil without causing cycloplegia?

1. Sphincter pupillae

2. Ciliary

3. Dilator pupillae

4. Radial

The correct answer is: Dilator pupillae

The correct choice refers to the dilator pupillae muscle, which is responsible for the widening of the pupil or mydriasis. When drugs that dilate the pupil take effect without affecting the ciliary muscle (which controls accommodation and is involved in cycloplegia), they primarily stimulate the dilator pupillae. Mydriatic agents like phenylephrine act specifically on the dilator pupillae by stimulating the sympathetic pathways, leading to the contraction of this muscle. This action causes the pupil to enlarge while leaving accommodation intact, as the ciliary muscle remains unaffected. In contrast, the sphincter pupillae muscle functions to constrict the pupil, and the ciliary muscle's role is connected with focusing and accommodation rather than dilation. The radial muscle is largely synonymous or sometimes used interchangeably with the dilator muscle, but in this context, dilator pupillae is the most precise term for the muscle that specifically promotes pupil dilation. Thus, the dilator pupillae is the muscle that responds to these mydriatic agents without causing cycloplegia, making it the best choice in this scenario.