Which fibrous protein of the intermediate filaments strengthens hair, nails and skin?

1. Intermediate filaments are the most diverse group of cytoskeletal elements. 2. Several types of fibrous protein are found among them. 3. The most familiar is keratin, which strengthens hair, nails and the epidermis of the skin. 4. Collagen is the abundant protein of the extracellular matrix instead, so it sits outside the cell. _Source: OpenStax Biology (1st ed., CC BY 4.0), Ch 4 "Cell Structure", section 4.5 The Cytoskeleton_