Which vitamin does your skin synthesize?

Correct Answer: C

Rationale: The skin plays a unique role in vitamin production, specifically synthesizing vitamin D when exposed to ultraviolet (UV) light from the sun. This process occurs in the epidermis, where UV rays convert 7-dehydrocholesterol into previtamin D3, which then becomes vitamin D3 (cholecalciferol). This vitamin is crucial for calcium absorption and bone health. Vitamin A, essential for skin cell development, is not synthesized by the skin but obtained through diet (e.g., from beta-carotene). Vitamin C, an antioxidant vital for collagen synthesis, is also diet-derived, not produced in the skin. Vitamin E, another antioxidant protecting skin cells, is similarly acquired through food or supplements, not manufactured by the integument. The skin's ability to produce vitamin D distinguishes it from other vitamins, an evolutionary adaptation to ensure adequate levels in sunlight-exposed populations. This synthesis is regulated by exposure time and skin pigmentation, with darker skin requiring more UV exposure due to melanin's filtering effect. Thus, vitamin D is the only vitamin the skin synthesizes, aligning with its physiological role.

Correct Answer: C

Rationale: Nails originate from the nail matrix, a specialized epidermal region beneath the proximal nail fold. The matrix contains actively dividing keratinocytes that produce the nail plate, a hard keratin structure, through keratinization. As cells proliferate, they push the nail forward over the nail bed, determining its growth and thickness. The eponychium, or cuticle, is the skin fold overlapping the nail's base, protecting the matrix but not forming the nail. The lunula, the white crescent at the nail's base, is part of the matrix visible through the thin nail, not the origin. 'Cuticle' often refers to the eponychium in common use, but anatomically, it's not the nail's source. The matrix's role is evident in nail regrowth after injury damage here alters nail shape, unlike other regions. Dermatological texts confirm the matrix as the nail's generative tissue, distinguishing it from surrounding protective or visible parts.

Correct Answer: C

Rationale: Full-thickness burns, which destroy both the epidermis and dermis, eliminate the skin's waterproof barrier, primarily the stratum corneum, leading to uncontrolled water loss from exposed tissues. This can cause severe dehydration and hypovolemic shock, making it life-threatening when over 20% of the body surface is affected. While vitamin D synthesis occurs in the skin, its loss isn't immediately fatal. Loss of sensation is a concern but not the primary threat. Abrasion of internal tissues happens but is secondary to fluid loss and infection risk (not listed but noted in the source). The critical issue is the skin's inability to retain water, a fundamental protective function, making this the key reason for the severity.

Correct Answer: D

Rationale: Keratinocytes produce keratin, forming the skin's tough outer layer correct. Merkel cells connect to sensory nerve endings for touch correct. Melanocytes produce melanin for pigmentation correct. Dendrocytes (likely meaning dendritic cells or macrophages in the dermis) don't produce 'dendrocidin,' a non-existent substance, making this incorrect. Dendritic cells process antigens for immunity, not a specific secretion like dendrocidin. The error lies in this fabricated term, inconsistent with skin cell functions, identifying it as the false statement.

Correct Answer: B

Rationale: The stratum corneum is the most superficial skin layer, forming the horny, keratinized surface of the epidermis that protects against abrasion and water loss. The stratum lucidum, found in thick skin, lies just beneath it. The papillary and reticular dermal layers are in the dermis, below the epidermis, with the papillary being more superficial but still deeper than the corneum. As the outermost barrier, constantly shedding and renewing, the stratum corneum holds the top position in skin structure, making it the clear choice.