Which structure controls the shape of the lens?

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

Answer: (B)

Accommodation is controlled by the ciliary muscle, a smooth muscle in the ciliary body that surrounds the lens. When the ciliary muscle contracts, it releases tension on the zonular fibers (suspensory ligaments), allowing the lens to become thicker and more curved to increase its optical power for near vision. When the muscle relaxes, the zonules pull on the lens, flattening it for distance vision. The iris is responsible for adjusting pupil size, the choroid provides vascular support behind the retina, and the retina itself is the light-sensitive layer that detects images. Understanding this helps you anticipate how a person may need to shift focus between objects at different distances during mobility tasks, and how aging can affect near vision through reduced accommodation (presbyopia).