Vitamin D is created from by skin cells.

Correct Answer: A

Rationale: Skin cells synthesize vitamin D from 7-dehydrocholesterol (often shortened to dehydrocholesterol) in the epidermis when exposed to UVB light, converting it to cholecalciferol (vitamin D3). Cholesterol is a related lipid but not the direct precursor in this process. Hydrocholesterol and hydrodermis are not real terms in this context; they seem to be distractors. Dehydrocholesterol is specifically stored in the skin and transformed by sunlight, making it the correct substance involved in vitamin D production.

Correct Answer: A

Rationale: The epidermis, the outermost skin layer, has specific characteristics that define its role as a protective barrier. It is not highly vascular; it lacks blood vessels entirely, relying on diffusion from the dermis for nutrients. This avascular nature distinguishes it from the dermis, which is rich in blood supply. The epidermis does contain melanin, produced by melanocytes, and keratin, formed as cells mature, both essential for protection and strength. It is distinctly stratified, with layers like the stratum germinativum, spinosum, granulosum, and corneum reflecting its cellular progression. It also gives rise to sebaceous and sweat glands, which develop as epidermal invaginations into the dermis. The false statement is the claim of vascularity, as the epidermis's lack of blood vessels is a fundamental feature, ensuring it remains a tough, renewable barrier while depending on underlying tissues for sustenance, a design critical to its function.

Correct Answer: C

Rationale: Alopecia, or hair loss, has multiple potential causes, but one predominates. Protein deficiencies can weaken hair, as keratin requires amino acids, but this is rare in well-nourished populations and not the leading cause. Dermal viral infections might cause localized hair loss (e.g., fungal infections like ringworm), but they are not primary alopecia triggers. Stress can induce temporary hair loss (telogen effluvium), where hair follicles enter a resting phase, but it's reversible and less common as a chronic cause. Genetic inheritance, particularly androgenetic alopecia (male or female pattern baldness), is the most probable and widespread cause, affecting millions globally. Driven by hormones (androgens) and genetics, it leads to progressive follicle miniaturization. Studies show its prevalence up to 50% of men and 25% of women by age 50 outstrips other causes. Its permanence and familial patterns distinguish it as the dominant etiology, aligning with clinical observations and epidemiological data.

Correct Answer: C

Rationale: The arrector pili are small muscles attached to hair follicles, controlled by the sympathetic nervous system. When stimulated (e.g., by cold or fear), they contract, pulling the hair upright and causing the skin to pucker, resulting in goose bumps (piloerection). They are not glands, so they don't secrete anything like sebum (which comes from sebaceous glands) or sweat (from sweat glands). Excessive sweating is linked to eccrine glands, not arrector pili. Their role in creating goose bumps is a well-known physiological response to sympathetic activation, conserved from animals raising fur for warmth or defense, making this the correct function.

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.