Which statement best describes keratin?

Keratin is a fibrous structural protein that plays a crucial role in the composition of the epidermis, hair, and nails. This protein is known for its resilience and protective properties; it forms a key component of the outer layer of the skin, providing toughness and waterproofing. In hair and nails, keratin contributes to their strength and durability, making them resistant to physical damage and environmental factors.

The other options do not accurately describe keratin. For instance, a soft protein found in muscles would not encompass the characteristics of keratin, as keratin is not involved in muscle structure. Additionally, keratin is not an enzyme; enzymes are biological catalysts, primarily made up of proteins, but they have different functions from structural proteins like keratin. Finally, describing keratin as a lipid is inaccurate, as lipids refer to a different class of organic compounds, which are fats and oils not related to the protein structure of keratin. Thus, the answer reflects the correct association of keratin with its presence in the epidermis, hair, and nails.