What is the primary structural protein that serves as a building block for hair, skin, and nails?

The primary structural protein that serves as a building block for hair, skin, and nails is keratin. Keratin is a fibrous protein that provides strength and resilience to these tissues. In hair, keratin forms the main structural component, contributing to its shape and durability. Similarly, in skin and nails, keratin helps to create a protective barrier and gives these tissues their firmness and resilience.

Collagen, while also important for skin structure, primarily provides support and elasticity in connective tissues rather than forming the main structural element in hair and nails. Elastin is another protein that helps tissues maintain their shape after stretching but does not play a direct role in the composition of hair, skin, or nails. Hemoglobin, on the other hand, is a protein found in red blood cells responsible for transporting oxygen, which is unrelated to the structural composition of hair, skin, or nails. Thus, keratin is specifically recognized for its critical role in forming and maintaining the integrity of these structures.