**Question:** Accommodation is due to

A. Chorioallantoic membrane
B. Ciliary muscle contraction
C. Iris constriction
D. Convergence of visual rays

**Correct Answer:** B. Ciliary muscle contraction

**Core Concept:** Accommodation is the process by which the eye adjusts its focal length to bring distant objects into clear focus on the retina. This is achieved through the contraction of the ciliary muscle, which results in the change of the shape of the lens within the eye.

**Why the Correct Answer is Right:** Ciliary muscle contraction is responsible for changing the shape of the lens in the eye. When the ciliary muscle contracts, it causes the zonules, which are fibrous cords that support the lens, to be pulled away from the lens. This results in a change in the curvature and thickness of the lens, which alters its refractive power and brings distant objects into clear focus on the retina.

**Why Each Wrong Option is Incorrect:**

A. Chorioallantoic membrane: This refers to the membrane that develops between the yolk sac and allantois in embryos. It is unrelated to accommodation in the adult eye.

C. Iris constriction: Iris constriction, or pupillary constriction, reduces the size of the pupil to control the amount of light entering the eye, but it does not directly contribute to accommodation.

D. Convergence of visual rays: Convergence is the process by which the eyes align to bring nearby objects into clear focus, not accommodation. Accommodation is focused on bringing distant objects into focus.

**Clinical Pearl:** Accommodation is an essential process for maintaining clear vision at varying distances, particularly when reading or looking at objects that are not placed at a fixed distance from the eye. Understanding the role of ciliary muscle contraction in changing the lens shape is crucial for grasping the concept of accommodation and its impact on visual acuity.