Which layer of the cornea comprises 90% of its thickness?

The stroma is indeed the correct choice, as it accounts for approximately 90% of the cornea's thickness. This layer is primarily made up of collagen fibers and water, giving the cornea its strength, structure, and clarity. The stroma is crucial in maintaining the overall shape of the cornea and plays a vital role in refracting light as it enters the eye. Its unique arrangement of collagen fibers allows for transparency while providing the necessary support for the epithelium and other layers above and below it.

Other layers of the cornea include the epithelium, which serves as a protective outer layer, the endothelium, which is responsible for maintaining corneal hydration, and Descemet's membrane, a thin layer that supports the endothelium. Though each of these layers has significant functions, none contribute to the corneal thickness to the extent that the stroma does. Understanding the structure and composition of the cornea is essential for appreciating how it functions in vision and overall eye health.