The outermost layer of the skin is called:

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.

Correct Answer: D

Rationale: Thermoreceptors, specialized nerve endings in the dermis and epidermis, detect temperature changes, signaling heat or cold to maintain homeostasis. Meissner's corpuscles sense light touch, Ruffini endings detect stretch and sustained pressure, and Merkel cells (via discs) sense continuous touch, none primarily temperature-focused. Thermoreceptors' sensitivity to thermal stimuli, distinct from mechanical or tactile roles, makes them the correct receptor type, essential for skin's sensory function.

Correct Answer: C

Rationale: Excretion via skin involves sweat glands eliminating waste like salts, urea, and water, a minor but real function alongside thermoregulation. Thermoregulation cools via sweat evaporation, sensation detects stimuli, and protection shields from harm, but only excretion directly addresses waste removal. This process, though secondary to kidney function, occurs through eccrine glands, making it the correct function.

Correct Answer: A

Rationale: The epidermis is primarily keratinocytes, epithelial cells that produce keratin, forming its stratified layers from basale to corneum, comprising about 90% of its cells. Fibroblasts, in the dermis, make collagen. Adipocytes, fat cells, dominate the hypodermis. Chondrocytes form cartilage, not skin. Keratinocytes' prevalence and role in barrier formation define the epidermis, making them the correct cell type.

Correct Answer: B

Rationale: Collagen, abundant in the dermis's reticular layer, provides strength and resilience, resisting stretching and supporting skin structure, produced by fibroblasts. Melanin is an epidermal pigment, keratin fills epidermal cells, and sebum is an oily secretion, none structural in the dermis. Collagen's fibrous network, paired with elastin, ensures skin durability, making it the main protein.