Which of the following is NOT one of the five main parts of a hair root?

The hair root consists of several critical components that contribute to the growth and health of the hair. Among these components are the follicle, which serves as the sheath surrounding the hair root; the arrector pili muscle, which is responsible for causing hair to stand on end (commonly known as goosebumps); and the sebaceous glands, which produce oil to keep the hair and scalp moisturized.

The hair shaft, however, is not part of the hair root itself. Instead, it is the visible part of the hair that extends above the scalp, formed from keratinized cells. While the shaft is crucial for hair appearance and styling, it does not play a role in the foundational structure or growth processes associated with the hair root. Thus, identifying the hair shaft as not belonging to the five main parts of the hair root highlights the distinction between the internal structure of hair and the external visible portion.