## Core Concept
Presbyopia is a age-related condition that affects the eye's ability to focus on close objects. It is a type of farsightedness that occurs due to a decrease in the flexibility of the lens and a weakening of the ciliary muscles. This condition typically starts in the early to mid-40s.

## Why the Correct Answer is Right
The correct answer, , is related to the loss of elasticity of the lens and a decrease in the power of accommodation. As we age, the lens becomes less flexible and the ciliary muscles that control the shape of the lens weaken. This makes it difficult for the eye to focus on close objects. The presbyopia is primarily caused by the **scleral rigidity** and **loss of lens elasticity**, which affects the eye's accommodation reflex.

## Why Each Wrong Option is Incorrect
* **Option A:** This option is incorrect because while changes in the cornea may occur with age, they are not the primary cause of presbyopia.
* **Option B:** This option is incorrect because the primary issue in presbyopia is not related to the **shape of the eyeball** but rather to the lens and its accommodating mechanism.
* **Option C:** This option might seem related but is not the best choice because it does not directly address the **mechanism** of presbyopia.

## Clinical Pearl / High-Yield Fact
A key point to remember is that presbyopia typically starts in the early to mid-40s and is corrected with reading glasses, bifocal glasses, or contact lenses. A classic clinical correlation is that presbyopia and cataracts often present around the same age, and patients may require surgery for cataracts and correction for presbyopia simultaneously.

## Correct Answer: D. Loss of elasticity of lens.