Certified Paraoptometric (CPO) Practice Exam 2025

The iris is the structure responsible for controlling the size of the pupil. It contains muscles that contract and relax to adjust the size of the pupil, thereby regulating the amount of light that enters the eye. When the lighting conditions change, the iris responds by either dilating (enlarging) the pupil in low light to allow more light in or constricting (shrinking) the pupil in bright light to reduce the amount of light entering the eye.

The ciliary muscle's primary function is to change the shape of the lens for focusing on objects at various distances, rather than controlling pupil size. The lens itself is responsible for focusing light onto the retina and does not have a role in pupil size regulation. The cornea provides overall structural support and is involved in the refraction of light but does not influence pupil size either. Thus, among the options presented, the iris is the only one that directly controls the size of the pupil.