NOCTI Cosmetology Practice Exam 2026 - Free Cosmetology Practice Questions and Study Guide

The correct answer is keratin, as it is the fundamental protein that serves as the primary structure of hair, providing strength and resilience. Hair is composed largely of keratin, and this fibrous protein forms the building blocks of the strands. Keratin contributes to the hair's durability and helps to protect it from environmental factors, thereby influencing the overall health and appearance of all hair types.

The cuticle, while important, is merely the outermost layer that protects the inner structure of the hair. It does not serve as the foundational element for all hair types. The cortex, located beneath the cuticle, provides the hair with its elasticity and strength, but it is still made up of keratin. The medulla is the innermost layer, which is present in some hair types but not all, and does not contribute significantly to the overall structure of hair in every individual. Thus, keratin is the essential component that serves as the foundation for all hair types.