What structural protein is primarily found in hair and nails?

Keratin is the primary structural protein found in hair and nails, providing both strength and resilience. It is a fibrous protein that is key to the structure of these components of the integumentary system. In hair, keratin not only contributes to its toughness but also plays a role in its appearance and texture. Similarly, in nails, keratin helps to form the hard protective layer that shields the underlying tissues.

Collagen, while an essential protein found throughout the body, including in the skin, tendons, and ligaments, serves primarily in providing structural support and elasticity rather than being the main component of hair and nails. Elastin is also involved in providing elasticity in various tissues but is not a significant part of both hair and nails. Melanin, on the other hand, is a pigment responsible for color in the skin, hair, and eyes, but it is not a structural protein. Thus, keratin is the correct answer as it directly relates to the composition and function of hair and nails in the integumentary system.