A cataract is the opacification of which structure of the eye?

A cataract specifically refers to the opacification of the crystalline lens of the eye. The crystalline lens is a transparent structure located behind the iris and the pupil, and its primary function is to focus light onto the retina. When this lens becomes cloudy or opaque, it disrupts the passage of light and leads to vision problems, such as blurred or dimmed sight. This condition is typically age-related but can also result from other factors, including diabetes, prolonged use of corticosteroids, or trauma.

The other structures listed—cornea, retina, and iris—serve different functions within the eye. The cornea is responsible for the initial focus of light as it enters the eye, the retina is the light-sensitive layer that converts light into neural signals for vision, and the iris regulates the amount of light that enters the eye by controlling the size of the pupil. None of these structures are associated with the development of cataracts. Therefore, the crystalline lens is the accurate response to identify where opacification occurs in cataract formation.