Which of the following pairings is appropriate?

Correct Answer: C

Rationale: Epidermal layers have distinct roles and compositions. The stratum germinativum is the basal layer where cell division occurs, producing cells that eventually keratinize, but it is not primarily associated with keratin itself. The stratum corneum, the outermost layer, is packed with keratin, not melanocytes, which are found in the stratum germinativum and produce melanin. The stratum granulosum is where keratinization begins, with cells accumulating keratin as they die and move upward, making it a fitting match for keratin. The stratum lucidum, present only in thick skin, consists of dead, keratinized cells but lacks blood vessels, which are exclusive to the dermis. The correct pairing hinges on the layer most directly tied to keratin production and accumulation. The stratum granulosum's role in this process forming keratohyalin granules and initiating the transition to the stratum corneum makes it the strongest match, aligning with histological descriptions of epidermal differentiation and keratin's protective function.

Correct Answer: C

Rationale: The dermis consists of two layers: the papillary (upper) and reticular (lower), both primarily made of connective tissue, including collagen and elastin fibers, which provide strength, elasticity, and support. The papillary layer has loose connective tissue with blood vessels and nerves, while the reticular layer is denser with thick collagen bundles. Melanocytes, which produce pigment, are found in the epidermis, not the dermis. Keratinocytes, the main epidermal cells, form the skin's outer layers, not the dermis. Adipose tissue is characteristic of the hypodermis, beneath the dermis, not within it. The connective tissue composition of the dermis is fundamental to its structural and protective roles, making it the correct answer.

Correct Answer: A

Rationale: Skin exposure to sunlight, specifically UVB rays, triggers vitamin D synthesis by converting 7-dehydrocholesterol in the epidermis to vitamin D3 (cholecalciferol), essential for calcium absorption and bone health. Arteriole constriction occurs in response to cold, not sunlight, to reduce heat loss. Folate production isn't directly tied to sunlight; in fact, UV light can degrade folate in the skin. Thermoregulation involves sweating and blood flow adjustments, not a sunlight requirement. Vitamin D synthesis is a well-established photochemical process in the skin, making it the correct sunlight-dependent function.

Correct Answer: B

Rationale: Wound healing follows clotting (hemostasis) with inflammation, then proliferation, where fibroblasts in the dermis increase production of connective tissue (collagen) to rebuild the wound bed and form granulation tissue. Melanin production by melanocytes affects pigmentation, not repair. Pacinian corpuscles sense pressure, not part of healing, and their number doesn't increase. The stratum lucidum, a thin layer in thick skin, doesn't 'activate' in healing. Connective tissue production is the next key step in the proliferative phase, laying the foundation for wound closure, making it the correct response.

Correct Answer: B

Rationale: Among skin cancers, malignant melanoma is the most deadly due to its aggressive nature and high metastatic potential. Originating in melanocytes, the pigment-producing cells in the epidermis, it can spread rapidly to lymph nodes, lungs, and brain if untreated, with a 5-year survival rate dropping significantly in advanced stages (e.g., below 30% for stage IV). Basal cell carcinoma, the most common skin cancer, arises from basal cells in the epidermis and rarely metastasizes, making it highly treatable with surgery. Squamous cell carcinoma, from squamous cells, can metastasize but does so less frequently and aggressively than melanoma, with better prognosis if caught early. Melanoma's lethality stems from its ability to invade deeper tissues and its link to UV exposure, which triggers mutations (e.g., in the BRAF gene). Statistics show it causes the majority of skin cancer deaths despite being less common, emphasizing its severity over the slower-growing, less invasive basal and squamous types.