Full thickness burns to more than 20% of the body surface is life-threatening because of the:

Correct Answer: D

Rationale: Full-thickness burns over 20% cause life-threatening fluid loss (dehydration) by destroying the waterproof skin barrier and allow infection by removing protection against pathogens. Vitamin D loss isn't acutely fatal. Thermoregulation fails but is secondary to fluid and infection risks. Skin doesn't significantly excrete lactic acid, urea, or uric acid (kidney roles). The dual crisis of fluid loss and infection vulnerability drives burn mortality, as clinical management focuses on fluids and antibiotics, making this the critical pair.

Correct Answer: C

Rationale: The skin regulates temperature via sweat glands and blood vessels, senses stimuli through receptors, and protects against pathogens with its physical barrier and immune cells, but hormone production isn't a primary skin function. Hormones like cortisol or testosterone are produced by endocrine glands (e.g., adrenals, gonads), not skin. While skin synthesizes vitamin D, a prohormone, this is a metabolic process, not hormone production in the endocrine sense. Temperature regulation involves sweating and vasodilation, sensation uses nerve endings, and pathogen protection relies on the stratum corneum and Langerhans cells. Hormone production's absence from skin's core roles distinguishes it as the incorrect function here.

Correct Answer: B

Rationale: The stratum corneum, the epidermis's outermost layer, is a barrier of dead, keratinized cells and lipids that prevents pathogen entry and water loss, protecting the body from infection and dehydration. Sensation involves dermal receptors, not this dead layer. Melanin is produced in the stratum basale, not corneum. Temperature regulation uses sweat and vessels, not this passive shield. Its tough, waterproof structure, constantly shedding to renew protection, defines its primary role, a cornerstone of skin defense.

Correct Answer: C

Rationale: Sebum, an oily secretion from sebaceous glands, lubricates skin and hair, preventing dryness and brittleness while adding a minor protective layer. UV protection is melanin's role, not sebum's. Temperature regulation involves sweat and vessels, not oil. Melanin production is melanocytes' task, not glands'. Sebum's lipid content, delivered via hair follicles, maintains skin suppleness and waterproofing, making lubrication its primary function, per skin physiology.

Correct Answer: B

Rationale: Melanin, produced by melanocytes, protects against UV radiation by absorbing it, preventing DNA damage and reducing skin cancer risk, darkening as a shield (tanning). Melatonin regulates sleep, not UV defense. Sebum lubricates, not protects from UV. Collagen provides structure, not photoprotection. Melanin's UV-absorbing property, distributed in the epidermis, is the skin's primary anti-radiation mechanism, making it the correct answer.