Which of the following is not a function of the hypodermis?

Correct Answer: C

Rationale: The hypodermis, or subcutaneous layer, is a fatty layer beneath the dermis that protects underlying organs by cushioning them, helps maintain body temperature through insulation, and serves as a long-term energy storage site via adipose tissue. However, it is not a source of blood vessels in the epidermis. The epidermis itself is avascular (lacking blood vessels), relying on diffusion from the papillary dermis, not the hypodermis, for nutrients. The hypodermis does contain larger blood vessels, but they supply the dermis, not directly the epidermis. This distinction in vascular anatomy excludes the hypodermis as a direct source for epidermal blood vessels, making it the correct choice for what is not a function.

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: B

Rationale: Skin color results from three key contributors: carotene, melanin, and hemoglobin. Melanin, produced by melanocytes in the epidermis, provides brown to black tones, varying by amount and type (eumelanin vs. pheomelanin), and protects against UV damage. Carotene, a dietary pigment, accumulates in the stratum corneum and hypodermis, adding yellowish hues, noticeable in lighter skin or excess intake. Hemoglobin, in dermal blood vessels, imparts pink or red tones based on oxygenation oxygenated blood brightens skin, while deoxygenated blood (cyanosis) dulls it. Keratin, a structural protein in the epidermis and hair, is colorless and waterproofs but doesn't pigment skin. Combining melanin's depth, carotene's tint, and hemoglobin's flush explains the full spectrum of human skin tones, from pale to dark, across populations. Omitting any of these misrepresents the complex interplay, with melanin dominant in darker skin and hemoglobin's effect more visible in lighter complexions, making this trio the accurate basis.

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.