Which of the following can pass most easily through the epidermis?

Correct Answer: B

Rationale: Lipid-soluble molecules pass most easily through the epidermis due to its structure. The stratum corneum, rich in keratin and lipids (e.g., ceramides, cholesterol), forms a hydrophobic barrier. Lipid-soluble substances, like steroids or fat-soluble vitamins (A, D, E, K), dissolve into this lipid matrix, diffusing through cell membranes and intercellular spaces. Proteins, large and often hydrophilic, cannot penetrate this barrier, remaining excluded unless via wounds. Water-soluble compounds, like glucose, struggle to cross without carriers, as the corneum repels water. Salts, ionic and water-soluble, face similar resistance, though sweat ducts allow minimal passage. Transdermal drug delivery exploits this, using lipid-based patches for absorption. The epidermis's avascularity and lipid composition favor nonpolar molecules, a principle backed by pharmacological and physiological research, distinguishing lipid-soluble penetration from polar or large substances.

Correct Answer: C

Rationale: Sudiferous glands are sweat glands with two types: eccrine, which secrete watery sweat through ducts directly to the skin surface for cooling, and apocrine, which secrete thicker sweat into hair follicles, often in areas like the armpits. Sebum is secreted by sebaceous glands, not sudiferous. Cerumen (earwax) comes from ceruminous glands, not sweat glands. The dual mechanism eccrine to the surface, apocrine to follicles defines sudiferous glands' function in thermoregulation and excretion, making this the comprehensive answer.

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: D

Rationale: Apocrine glands, a type of sudiferous gland, secrete a thicker, milky sweat into hair follicles, often in areas like the armpits, triggered by stress or hormones. 'Apocrin' is a nonsense term. Cerumin (cerumen) is earwax from ceruminous glands. Milk comes from mammary glands, not apocrine. As sweat glands, apocrine glands differ from eccrine by their secretion site and composition, contributing to body odor when broken down by bacteria, making sweat the accurate secretion.

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.