A 50-year-old man came to the ophthalmologist with the complaint of inability to read newspaper. His vision problem is likely due to inadequate contraction of which of the following structures?
**Core Concept**
The patient's complaint of inability to read newspaper suggests a problem with near vision, which is primarily mediated by accommodation. The process of accommodation involves the contraction of the ciliary muscles, leading to a decrease in the thickness of the lens and an increase in its curvature, thereby increasing the refractive power of the lens and allowing for near vision.
**Why the Correct Answer is Right**
The contraction of the ciliary muscles is essential for the process of accommodation, which is the mechanism by which the eye focuses on objects at near distance. When the ciliary muscles contract, they release tension on the zonular fibers, allowing the lens to become more spherical and increasing its refractive power. This is a crucial mechanism for near vision, which is often impaired in conditions such as presbyopia, where the lens becomes less flexible with age. The ciliary muscles play a key role in this process, and their contraction is necessary for adequate accommodation.
**Why Each Wrong Option is Incorrect**
* **Option A:** The extraocular muscles are responsible for eye movements, such as rotation and version, but they do not play a direct role in accommodation or near vision.
* **Option B:** The iris is responsible for regulating the amount of light that enters the eye by adjusting the size of the pupil, but it does not contribute to the process of accommodation.
* **Option D:** The optic nerve is responsible for transmitting visual information from the eye to the brain, but it does not play a direct role in the process of accommodation.
**Clinical Pearl / High-Yield Fact**
Presbyopia, a common condition associated with aging, is characterized by an inability to focus on near objects due to decreased flexibility of the lens. This is often managed with the use of reading glasses or contact lenses that provide additional refractive power for near vision.
**Correct Answer:** C. ciliary muscles. Ciliary muscles.