Keratin is primarily found in which of the following?

Keratin is a fibrous structural protein that is a key component of the outer layer of the skin (epidermis), as well as hair and nails. Its primary role is to provide strength and protection from environmental damage, pathogens, and dehydration. In the epidermis, keratin forms a barrier that helps to retain moisture and prevent water loss from the underlying tissues. In hair and nails, keratin contributes to their rigidity and resilience.

The other options do not primarily contain keratin. Mucous membranes are composed mainly of epithelial cells and are involved in secretion and absorption, not keratin production. Sweat glands are primarily glandular structures that produce sweat and do not significantly contain keratin. Blood vessels are made of smooth muscle cells and connective tissue, lacking keratin altogether. Therefore, the primary location where keratin is found is in the epidermis, hair, and nails.