Final answer:
The lower segment of the hair follicle that includes the base and extends to the arrector pili muscle is the bulb. The correct option is A.
Step-by-step explanation:
The lower segment of the hair follicle that extends from the follicle base to the insertion of the arrector pili muscle is known as the bulb.
The bulb is a structure at the base of the hair root that surrounds the dermal papilla, which is a mass of connective tissue, blood capillaries, and nerve endings.
The hair bulb is also where the hair matrix is located; this is a layer of basal cells where a strand of hair grows from. The other options given, such as the cuticle, isthmus, and infundibulum, represent different parts of the hair structure.
The cuticle is the protective outer layer of the hair shaft. The isthmus is a region of the follicle above the bulb and below the opening of the sebaceous glands.
The infundibulum is the upper part of the follicle that extends from the skin's surface to the opening of the sebaceous gland. The correct option is A.