Which of the following is NOT one of the three main structures of hair?

The three main structures of hair are the medulla, cortex, and cuticle. These components play significant roles in the composition and characteristics of hair.

The medulla is the innermost layer of hair and can vary in thickness and presence depending on the type of hair. It is often absent in finer hair types. The cortex forms the middle layer, containing fibrous proteins and melanin, which give hair its strength, color, and texture. The cuticle is the outermost layer, made up of overlapping cells that protect the inner layers and contribute to the hair's shine and smoothness.

The inclusion of "fibula" in the options is what makes it the correct response in this question. The fibula is a bone in the leg, unrelated to hair structure, and therefore is not one of the three main components of hair anatomy. This distinction solidifies the understanding of hair's biological structure and its relevance in cosmetology practices.