In humans, exposure of the skin to sunlight is required for

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: 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.

Correct Answer: B

Rationale: Sebaceous glands, found in the dermis near hair follicles, produce sebum, an oily substance. Sebum, a mix of lipids like triglycerides and wax esters, lubricates hair and skin, preventing dryness and offering minor antimicrobial protection. It's secreted via a holocrine process cells disintegrate to release it into follicles, then to the skin surface. Milk is produced by mammary glands, modified sweat glands, not sebaceous ones. Sweat comes from eccrine and apocrine glands, distinct from sebaceous function, with water and salts, not oil. Cerumen (earwax) is made by ceruminous glands in the ear canal, a specialized sebaceous type, but not typical skin sebaceous glands. Sebum's oily nature distinguishes it, supporting skin flexibility and waterproofing, a role confirmed by histological and physiological studies of gland activity, setting it apart from other secretions.

Correct Answer: C

Rationale: Sebaceous glands are located in the dermis, the thick layer of skin beneath the epidermis, where they produce sebum, an oily substance that lubricates hair and skin. They are typically associated with hair follicles, which are embedded in the dermis, and secrete sebum into the follicle to reach the surface. The digestive system is unrelated to skin glands, ruling it out entirely. The hypodermis, or subcutaneous layer, contains fat and connective tissue, not sebaceous glands. The stratum corneum, the outermost epidermal layer, consists of dead, keratinized cells and lacks glands or living structures. Anatomical studies of skin consistently place sebaceous glands in the dermis, making this the correct location due to their functional and structural integration with dermal components like hair follicles and blood vessels.

Correct Answer: A

Rationale: Sudiferous glands are sweat glands (eccrine and apocrine), secreting sweat, not sebum, which is an oily substance produced by sebaceous glands. Sebaceous glands correctly secrete oil (sebum) to lubricate skin and hair. Apocrine glands, a type of sudiferous gland, secrete sweat into hair follicles. Ceruminous glands in the ear canal secrete cerumen (earwax). The incorrect statement is that sudiferous glands secrete sebum, as their function is sweat production, a distinction clear in skin physiology, making this the error to identify.