What must be the case for a drug to be administered transdermally?

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: 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.

Correct Answer: D

Rationale: The skin protects, secretes sebum, aids immunity, regulates temperature, excretes sweat, synthesizes vitamin D, perceives stimuli, and protects all true functions. It doesn't synthesize vitamin E, which is dietary, not skin-produced. 'Social function' (e.g., appearance) is a byproduct, not a primary physiological role like temperature regulation. The inclusion of vitamin E synthesis, an incorrect function, makes this list the one with non-integumentary roles.

Correct Answer: D

Rationale: The stratum corneum, the outermost epidermal layer, protects against water loss and abrasion with its dead, keratin-filled cells and lipid barriers. The stratum germinativum (basale) generates cells, not protection. The lucidum, in thick skin, is a transitional layer, not the main shield. The corneum's role as the skin's durable, waterproof surface, constantly abraded and renewed, makes it the critical protective stratum, per epidermal function.

Correct Answer: A

Rationale: The epidermis is the outermost layer of the skin, forming a protective barrier against the environment with its stratified squamous epithelium, primarily composed of keratinocytes and topped by the stratum corneum. The dermis lies beneath it, containing connective tissue and glands, not the surface. The hypodermis, also called subcutaneous tissue, is deeper still, beneath the dermis, and consists of fat and connective tissue, not the outermost layer. While 'subcutaneous tissue' is synonymous with hypodermis, it's not the skin's surface. The epidermis's position as the topmost layer, shielding against pathogens and dehydration, makes it the correct answer, consistent with skin histology.