What is the primary substance that composes a layer of the epidermis and protects the body from fluid loss?

The primary substance that composes a layer of the epidermis and protects the body from fluid loss is keratin. Keratin is a fibrous protein that forms a crucial component of the skin, hair, and nails. In the epidermis, particularly in the outermost layer known as the stratum corneum, keratin serves several essential functions. It helps create a waterproof barrier that minimizes water loss from the body and protects against various environmental factors, including pathogens and physical abrasions. This barrier function is vital because it helps maintain hydration and overall skin integrity.

Other substances mentioned, such as collagen and elastin, play important roles in the dermis, which is the layer beneath the epidermis, contributing to the skin's structural support and elasticity. Melanin, while important for pigmentation and protection against UV radiation, does not have a direct role in fluid retention or protection against fluid loss. Thus, keratin’s role in maintaining a waterproof barrier makes it the primary substance responsible for protecting the body from fluid loss in the epidermis.