Which structure is the ring of muscle tissue that forms the colored portion of the eye around the pupil and controls the size of the pupil opening?

• A. Iris
• B. Ciliary muscle
• C. Choroid
• D. Retina

Answer: (A)

The iris is the ring of muscle around the pupil that determines its size. It forms the colored portion you see around the pupil and contains two muscle groups: the sphincter pupillae, which constricts the pupil in bright light to make the opening smaller, and the dilator pupillae, which dilates the pupil in dim light to let in more light. This adjustment helps regulate how much light reaches the retina and supports proper vision. The ciliary muscle changes the lens shape for focusing, not pupil size; the choroid is the vascular layer supplying the retina, and the retina is the light-sensitive layer at the back of the eye.