A protein substance found in nails and hair is:

• A. Keratin
• B. Collagen
• C. Elastin
• D. Melanin

Answer: (A)

The building block common to both nails and hair is keratin, a tough, fibrous protein. Keratin’s strength comes from cross-links formed by sulfur-containing amino acids like cysteine, which create disulfide bonds that harden the protein and give nails and hair their rigidity and resilience. This makes nails hard and hair strong yet flexible. Collagen supports connective tissues and skin but isn’t the main ingredient in nails or hair. Elastin adds elasticity to various tissues but isn’t the primary structural protein there. Melanin is the pigment that colors hair and skin, not a structural protein. So keratin is the substance you’re looking for.