The skin participates in the production of vitamin D when which of the following occurs? When

Correct Answer: C

Rationale: Skin produces vitamin D when exposed to UVB radiation, converting 7-dehydrocholesterol in the epidermis to previtamin D3, which becomes vitamin D3 (cholecalciferol). Calcium presence doesn't trigger this; it's a downstream effect of vitamin D. PTH signals the kidneys, not skin, to activate vitamin D later. The liver modifies the skin's product, adding a hydroxyl group, but doesn't initiate it. UV exposure is the critical first step, a photochemical reaction unique to skin, making this the correct condition.

Correct Answer: C

Rationale: Transdermal drugs are applied to the epidermis (e.g., via patches) and absorbed through the skin into the bloodstream for systemic effects, not limited to local action (topical). Muscle injection is intramuscular, and anal insertion is rectal, both unrelated. Application to the epidermis initiates the transdermal process, relying on diffusion through skin layers, a common delivery method in pharmacology, making this the precise definition.

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: B

Rationale: Normal flora (bacteria) reside on the epidermis, particularly the stratum corneum, the skin's surface exposed to the environment. The dermis, beneath, is internal and sterile unless breached. 'Microdermis' isn't a term; likely a typo. The hypodermis, deeper still, isn't a flora habitat. The epidermis, as the outermost layer, hosts these microbes, which thrive without penetrating living tissue, making it the correct location.

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.