Which structure changes size in response to light levels, functioning as the eye's aperture?

• A. Pseudoguttae
• B. Pterygium
• C. Retina
• D. Pupil

Answer: (D)

The structure that changes size in response to light and acts as the eye’s aperture is the pupil. It is the opening in the iris whose diameter is adjusted by two iris muscles: the sphincter pupillae constricts the pupil in bright light (making the aperture smaller) and the dilator pupillae enlarges it in dim light (making the aperture larger). This dynamic control helps regulate how much light reaches the retina, protecting it in bright conditions and enhancing vision in the dark.

Other options don’t serve this function: pseudoguttae are not involved in light entry, pterygium is a growth of conjunctiva onto the cornea, and the retina is the light-detecting layer at the back of the eye, not the adjustable opening.