What is the primary substance that makes up the composition of nails?

The primary substance that makes up the composition of nails is keratin. Keratin is a tough, fibrous protein that is not only a major component of nails but also of hair and the outer layer of skin. Its structure is crucial for providing strength and resilience, which protects the underlying tissues and helps maintain the integrity of the nail.

Keratin is unique because it forms a protective barrier that is resistant to wear and damage, making it ideal for the functional role nails play in protecting the tips of fingers and toes, enhancing sensory perception, and assisting in gripping objects. The presence of keratin is what gives nails their hardness and durability.

In contrast, while collagen is a vital protein in the body and contributes to the structure of skin and connective tissues, it does not make up the composition of nails. Calcium is important for bone health and plays a role in other bodily functions, but it is not a primary component of nails. Fibrin is a protein involved in the blood clotting process and has no relation to the structure of nails. Therefore, keratin is the correct answer as it is the fundamental protein that constitutes nails.