Which one of the following cell types is responsible for forming the skin's ability to tan on exposure to sunlight?

Correct Answer: A

Rationale: Melanocytes in the stratum basale produce melanin, the pigment that darkens skin (tanning) upon sunlight exposure, absorbing UV to protect cell nuclei. Keratinocytes form keratin, not pigment, though they receive melanin. Dendrocytes (likely dendritic cells) handle immunity, not tanning. Lymphocytes, immune cells, aren't skin-based or pigment-related. Melanocytes' UV-responsive melanin production is the mechanism behind tanning, a protective adaptation, making them the correct cell type.

Correct Answer: D

Rationale: The skin is the largest organ in the human body, covering an average surface area of about 1.5 to 2 square meters in adults and weighing approximately 4-5 kilograms, including the epidermis, dermis, and hypodermis. It surpasses the heart, liver, and lungs in both size and mass when considered as a whole organ system. The heart, though vital, is a small muscular organ, roughly 300 grams. The liver, at about 1.5 kilograms, is significant but smaller in surface area. The lungs, while expansive internally, have a combined weight of around 1 kilogram and less external coverage than skin. The skin's extensive role in protection, sensation, and thermoregulation, coupled with its sheer physical extent across the body, confirms it as the largest organ, a fact well-established in anatomy.

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: B

Rationale: The subcutaneous tissue, or hypodermis, primarily contains adipose tissue, fat cells that insulate and store energy, cushioning underlying structures. Blood vessels and sweat glands are mostly in the dermis, though some vessels traverse the hypodermis. Collagen fibers dominate the dermis, not the hypodermis, which has looser connective tissue. Melanocytes are epidermal, not subcutaneous. Adipose tissue's prevalence defines the hypodermis's role, distinguishing it from skin layers above, making this the correct content.

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.