## **Core Concept**
The cornea is the transparent, avascular (without blood vessels) part of the eye that covers the front portion of the eye. It is composed of several layers, with the outermost layer being the epithelium and the innermost layer being the endothelium. The stroma, which makes up about 90% of the cornea, provides most of its strength and elasticity.

## **Why the Correct Answer is Right**
The correct answer, **D. Keratan sulfate and Chondroitin sulfate**, highlights the importance of glycosaminoglycans (GAGs) in the cornea. Keratan sulfate and chondroitin sulfate are types of GAGs that are present in the cornea and contribute to its elasticity and transparency. These molecules are components of proteoglycans, which are crucial for maintaining the structural integrity and hydration of the corneal stroma.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because it does not specify the correct components related to corneal elasticity.
- **Option B:** This option is incorrect as it does not accurately represent the substances responsible for corneal elasticity.
- **Option C:** This option is incorrect because, although it mentions a type of GAG, it does not accurately complete the correct pair of substances responsible for corneal elasticity.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that the cornea's unique composition, including its high concentration of **keratan sulfate and chondroitin sulfate**, is essential for its transparency and elasticity. This is crucial for the cornea's function in refracting light properly.

## **Correct Answer:** . **Keratan sulfate and Chondroitin sulfate**