The waterproof coating found in epidermal cells is called:

Correct Answer: B

Rationale: The waterproof coating in epidermal cells is keratin, a fibrous protein forming the stratum corneum, the epidermis's outermost layer. Produced during keratinization, keratin fills dead keratinocytes, creating a tough, insoluble barrier that prevents water loss and entry. This hydrophobicity, due to keratin's sulfur-rich structure and tight packing, protects against dehydration and environmental damage, a key adaptation for terrestrial life. Myelin, a lipid sheath, insulates nerves, not skin cells. Melanin, a pigment from melanocytes, absorbs UV light but doesn't waterproof. Albumin, a blood protein, maintains osmotic pressure, unrelated to epidermal coating. Keratin's dominance in the corneum up to 85% of its dry weight ensures the skin's impermeability, a fact confirmed by studies of skin barrier function, setting it apart from pigments or unrelated proteins.

Correct Answer: B

Rationale: Full-thickness skin burns destroy the epidermis and dermis, removing the waterproof stratum corneum and exposing underlying tissues, leading to rapid fluid loss (dehydration) and vulnerability to infection without the skin barrier. Stomach ulcers cause internal bleeding, not external fluid loss or skin infection risk. Muscle tears and bone fractures may bleed or infect, but they don't typically cause massive fluid loss like burns, as the skin remains intact. Burns' dual threat of fluid loss and pathogen entry, especially over large areas, makes them uniquely life-threatening, as seen in burn unit protocols.

Correct Answer: B

Rationale: Transdermal drugs, applied via patches, must penetrate the skin's lipid-rich stratum corneum, requiring lipid solubility to pass between keratinized cells and reach the bloodstream. Water-soluble drugs struggle to cross this waterproof barrier. Subcutaneous injection bypasses the skin, not transdermal. Chemo-receptor irritation is unrelated to transdermal delivery, which focuses on absorption, not sensory response. Lipid solubility, often enhanced by chemical formulation, is a pharmacological necessity for transdermal success, as seen in drugs like nicotine patches, making this the key requirement.

Correct Answer: A

Rationale: Vitamin D production starts in the skin when UVB radiation converts 7-dehydrocholesterol in the epidermis to previtamin D3, initiating the process. PTH signals the kidneys to activate vitamin D later, not the skin. Calcium presence is an outcome, not a trigger. Hydroxylation occurs in the liver and kidneys, not the skin, modifying the skin's product. UV exposure is the essential first step, a photochemical reaction unique to epidermal cells, making this the correct initiating condition.

Correct Answer: D

Rationale: The stratum corneum, the epidermis's outermost layer, consists of dead, keratinized cells that protect against heat, chemicals, and bacteria, forming a durable barrier. Rapidly dividing cells are in the stratum basale, not corneum. Anchoring to muscle occurs via the hypodermis, not corneum. Collagen, elastin, and reticular fibers are in the dermis, not the avascular epidermis. The corneum's protective role, shedding pathogens and resisting environmental damage, is its primary function, making this the accurate description.