What other body parts are made of keratin aside from nails?

Keratin is a fibrous protein that plays a crucial role in protecting various structures in the body, providing strength and resilience. In addition to nails, hair and skin are other significant body parts that contain keratin.

Hair is primarily made of keratin, which forms the structure of each strand, making it strong and protecting the underlying cells. Similarly, the outer layer of the skin, known as the epidermis, is rich in keratinized cells that contribute to the skin's barrier function. This keratinized layer protects against environmental damage, prevents moisture loss, and provides defense against pathogens.

The other options mentioned do not consist of keratin. Muscles are made of muscle fibers and proteins like actin and myosin. Tendons, which connect muscles to bones, are primarily composed of collagen, not keratin. Cartilage, found in joints and other areas, has a different composition as well, mainly containing collagen fibers and a gel-like substance, chondrin. Thus, the best answer that correctly identifies other body parts made of keratin alongside nails is hair and skin.