As you are walking down the beach, you see a dead, dry, shriveled-up fish. Which layer of your epidermis keeps you from drying out?

Correct Answer: A

Rationale: The stratum corneum, the outermost epidermal layer, is composed of dead, keratinized cells that form a waterproof barrier, preventing water loss and keeping the body from drying out. The stratum basale generates new cells but doesn't block water loss. The stratum spinosum provides strength, not a moisture barrier. The stratum granulosum begins keratinization, but the corneum completes it, acting as the skin's primary shield against dehydration. This layer's lipid-rich, tough structure is why humans don't shrivel like the fish, making it the key protective layer.

Correct Answer: B

Rationale: Acne arises from multiple factors, but excess sebum is a primary cause. Sebaceous glands produce sebum, an oil that lubricates skin and hair. Overproduction, often triggered by hormonal changes (e.g., androgens in puberty), clogs hair follicles, creating a breeding ground for Propionibacterium acnes bacteria. This leads to inflammation, forming pimples. Hypersecretion of hormones, like testosterone, drives this excess sebum, making it a linked cause, though the question allows single selection, prioritizing sebum's direct role. Excess carotene affects skin color, not oil production or acne. Poor hygiene ('wash your face') can worsen acne by allowing bacterial buildup, but it's not the root cause overwashing may even irritate skin. Bacterial infection is a secondary factor after sebum clogs pores. Dermatological consensus highlights sebum's role in acne pathogenesis, with hormonal influence as a trigger, distinguishing it from dietary pigments or hygiene alone as the most immediate explanation.

Correct Answer: B

Rationale: Skin fibers, primarily collagen in the dermis, are arranged in bundles called lines of cleavage (or Langer's lines). These parallel alignments reflect the natural tension and orientation of collagen and elastic fibers in the reticular dermis, giving skin strength and flexibility. Surgeons use these lines for incisions, as cuts along them heal with less scarring due to minimal fiber disruption. The zone of hyperaemia is a region of increased blood flow, not a fiber structure. Epidermal ridges, on the epidermis, form fingerprints from dermal papillae, not fiber bundles. The stratum lucidum, an epidermal layer in thick skin, consists of dead cells, not fibers. Lines of cleavage are a dermal feature, visible in histological sections as organized collagen, critical for skin's mechanical properties and surgical outcomes, distinguishing them from epidermal or vascular terms.

Correct Answer: C

Rationale: The stratum corneum, the outermost epidermal layer, is composed of dead, keratin-filled cells packed tightly together, with glycolipids in the extracellular spaces acting as a waterproof sealant. This structure makes it a robust barrier against water loss, pathogens, and abrasion. Rapid division of melanocytes and keratinocytes occurs in the stratum basale, not the corneum, which is avascular and lacks blood capillaries those are in the dermis. Epidermal ridges, forming fingerprints, are in the dermis and basal epidermis, not the corneum. The keratin and glycolipid composition is a defining feature, critical to its protective role, distinguishing it from deeper, living layers.

Correct Answer: C

Rationale: The stratum germinativum, also called the stratum basale, is the deepest epidermal layer where keratinocytes rapidly divide via mitosis to replenish the epidermis, replacing cells lost from the surface. The stratum lucidum, in thick skin, is a thin, dead layer above the basale, not dividing. The papillary and reticular dermal layers are in the dermis, containing connective tissue, not keratinocytes. The germinativum's name (from 'germinate') reflects its role as the regenerative base, a key feature of epidermal renewal confirmed by histological studies.