Which fibrous protein is known as the building block for hair, skin, and nails?

The building block for hair, skin, and nails is keratin, which is a fibrous protein that provides structure and strength to these tissues. Keratin is particularly important in these areas due to its durability and protective qualities. In hair, keratin forms the primary structure of each strand, contributing to its resilience and ability to withstand various external factors. Similarly, in skin, keratin helps form a protective barrier against environmental damage, while in nails, it contributes to their hardness and protective features.

Collagen is also a crucial protein but serves different functions, primarily contributing to the structure and elasticity of connective tissues rather than specifically forming the core structure of hair, skin, and nails. Elastin is responsible for the elasticity of tissues and is not the main component in hair, skin, or nails. The term "proteins" is too broad and does not specify the particular protein involved in the structure of hair, skin, and nails. Therefore, keratin is the most accurate answer in the context of hair, skin, and nails due to its specific role as the primary protective structural protein.