What is the primary function of the arrector pili muscles associated with hair follicles?

Correct Answer: D

Rationale: Arrector pili muscles, small smooth muscles in the dermis attached to hair follicles, contract under sympathetic nervous stimulation (e.g., cold or fear), pulling hairs upright and causing goosebumps (piloerection). They don't sense temperature thermoreceptors do that. Melanin is from melanocytes, not muscles. 'Ejecting hair shafts' isn't a function; hairs grow or shed via follicles, not muscle action. Goosebumps, an evolutionary remnant to trap air or signal emotion, define their primary role, per skin anatomy.

Correct Answer: A

Rationale: Merkel cells, in the stratum basale, connect to nerve endings (Merkel discs) to detect sustained pressure and touch, providing fine tactile discrimination. Temperature is sensed by thermoreceptors, pain by nociceptors, and itch by specific receptors, none primarily Merkel-related. Their role in continuous pressure sensation, critical for texture perception, distinguishes them in the skin's sensory array, making this the correct sensation.

Correct Answer: C

Rationale: Lubrication and protection involve sebum production by sebaceous glands, moisturizing skin and hair while adding a protective layer against microbes and dryness. Thermoregulation uses sweat, sensation involves receptors, and vitamin D synthesis uses UV, none tied to oil. Sebum's role in maintaining skin integrity and suppleness defines this function, making it the correct answer.

Correct Answer: D

Rationale: The hypodermis insulates via adipose tissue, reducing heat loss, and stores energy as fat reserves, supporting survival during scarcity. Sweat production and temperature regulation (via vessels) occur in the dermis. The physical barrier is the epidermis's role. Insulation and energy storage define the hypodermis's primary function, cushioning and sustaining the body, making this the correct answer.

Correct Answer: C

Rationale: The epidermis is primarily keratinocytes, stratified epithelial cells that produce keratin, forming its layers from basale to corneum, about 90% of its composition. Fibroblasts are dermal, melanocytes produce melanin (a minority), and sebocytes (in sebaceous glands) are dermal, not epidermal. Keratinocytes' dominance and barrier role define the epidermis, making them the correct cell type.