Skin color is due to:

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: 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.

Correct Answer: B

Rationale: Vitamin D, as calcitriol, is crucial for calcium absorption in the gut, enhancing intestinal uptake to maintain blood calcium levels for bone health and muscle function. Haemostasis (clotting) relies on calcium but not directly on vitamin D production. Erythropoiesis (red blood cell formation) needs other factors like erythropoietin, not vitamin D. Intrinsic factor aids vitamin B12 absorption, unrelated to vitamin D. The skin-initiated, liver- and kidney-activated vitamin D process targets calcium regulation, a primary physiological role, making this the correct purpose.