COMT Practice Exam 2025 – Complete Resource for Exam Prep

The muscle responsible for pupil dilation is the dilator pupillae. This muscle is composed of radial fibers that, when contracted, pull the iris outward and enlarge the pupil. The dilator pupillae is under sympathetic nervous control, which is activated in response to low light conditions or stress, leading to an increase in pupil size to allow more light to enter the eye.

In contrast, the sphincter pupillae is involved in pupil constriction. This muscle is controlled by the parasympathetic nervous system and tightens to reduce pupil size. The ciliary muscle primarily focuses on accommodation, altering the shape of the lens for near or distant vision, rather than influencing the pupil's size directly. The rectus muscles are responsible for eye movement rather than pupil size.

Thus, the dilator pupillae is the correct choice because its contraction specifically leads to the dilation of the pupil, facilitating better vision in dim lighting and enhancing visual clarity.