AAMC FL Practice Exam

Which structure in the eye controls the size of the pupil opening?

1. The retina

2. The fovea

3. The lens

4. The iris

The correct answer is: The iris

The iris is the structure in the eye that controls the size of the pupil opening. It is a thin, circular structure made of muscle fibers and is located between the cornea and the lens. The iris can contract or relax, leading to changes in the diameter of the pupil, which regulates the amount of light that enters the eye. When the light is bright, the iris constricts the pupil to reduce light intake and protect the retina from excessive brightness. Conversely, in dim light conditions, the iris dilates the pupil to allow more light to enter, enhancing vision in low-light situations. This ability of the iris to change pupil size is crucial for maintaining optimal visual acuity and protecting the internal structures of the eye. The other structures mentioned do not control pupil size. The retina is responsible for converting light into neural signals, the fovea is a small depression in the retina that provides the clearest vision, and the lens focuses light onto the retina. Thus, the correct choice that directly relates to controlling pupil size is indeed the iris.