## **Core Concept**
The lens of the eye is composed of transparent, flexible tissue that changes shape to focus light on the retina. It is made up of **lens fibers**, which are produced throughout life by the lens epithelium. The lens grows by the addition of new layers of lens fibers, causing the older cells to be compacted towards the center.

## **Why the Correct Answer is Right**
The correct answer, , refers to the **nucleus** of the lens. The nucleus is the central, oldest part of the lens, composed of the most compact and oldest lens fibers. These cells have lost their organelles, including their nuclei, and are densely packed, contributing to the transparency and hardness of the nucleus. The nucleus grows very little after adolescence, and its cells are not replaced or regenerated.

## **Why Each Wrong Option is Incorrect**
* **Option A:** The **cortex** of the lens is made up of newer lens fibers than the nucleus. It surrounds the nucleus and is composed of **softer, more flexible** lens fibers that can be replaced or regenerated throughout life.
* **Option B:** The **epithelium** is a single layer of cells on the anterior surface of the lens and is responsible for producing new lens fibers. These cells are not the oldest part of the lens.
* **Option D:** The **capsule** is a transparent, elastic basement membrane that surrounds the lens and provides attachment for the zonular fibers. It does not contain cells.

## **Clinical Pearl / High-Yield Fact**
The lens nucleus becomes **harder and less flexible** with age, which can affect the eye's ability to focus on close objects (presbyopia). This hardening of the nucleus is also a factor in the development of **nuclear cataracts**, which are a common age-related condition.

## **Correct Answer:** . Nucleus