Which layer is a tattoo's ink injected into?

Correct Answer: B

Rationale: Tattoo ink is injected into the dermis, the thick layer beneath the epidermis. The epidermis, the outermost layer, constantly renews itself, shedding dead cells from the stratum corneum every few weeks. If ink were placed here, it would fade quickly as cells slough off. The dermis, however, is stable, containing collagen, blood vessels, and nerves, and does not regenerate rapidly. Tattoo needles penetrate about 1-2 mm deep, depositing ink into this layer, where it remains trapped by fibroblasts and immune cells, ensuring permanence. The hypodermis (or subcutaneous layer, also listed as 'SubQ') lies deeper, storing fat, and is too far below the surface for tattoo visibility or precision. Injecting ink there would blur the design and miss the dermis's ideal depth. The dermis's vascularity can cause initial bleeding, but its stationary nature preserves the tattoo long-term, a fact exploited by tattoo artists globally, confirming it as the target layer.

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: B

Rationale: Sensory receptors in the skin detect stimuli like touch, pressure, or pain. Meissner corpuscles sense light touch, root hair plexuses detect hair movement, and nociceptors register pain all are receptors in the dermis or epidermis. Apocrine glands, however, are sweat glands associated with hair follicles, secreting sweat in response to stress or hormones, not sensing stimuli. They're part of the skin's excretory system, not its sensory network. This functional distinction excludes apocrine glands from being sensory receptors, aligning with their role in secretion rather than perception.

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.