What is keratin primarily found in?

Keratin is a fibrous structural protein that plays a crucial role in forming the outer layer of skin, as well as being a key component of hair and nails. It provides strength, protection, and resilience to these tissues, making them resistant to damage and environmental factors. The epidermis, which is the outermost layer of skin, contains keratinocytes that produce keratin, providing the skin with its protective properties. Similarly, hair and nails are primarily made of keratin, contributing to their structure and durability.

In contrast, other options involve tissues that do not contain keratin as a primary component. Muscles and ligaments mainly consist of muscle fibers and connective tissue, while blood and plasma are composed of various cells and proteins such as hemoglobin and fibrinogen. The brain and nervous tissue are primarily made up of neurons and glial cells, lacking significant keratin content. Therefore, the presence of keratin in the epidermis, hair, and nails makes this the correct choice.