The skin accounts for what percentage of the body weight?

Correct Answer: D

Rationale: The skin is the largest organ of the human body, and its weight relative to total body weight is a well-established fact in anatomy. It constitutes approximately 7% of an average person's body weight, though this can vary slightly depending on factors like hydration, fat content, and individual size. For a 70 kg (154 lb) person, this equates to about 4.9 kg (11 lb) of skin, including the epidermis, dermis, and hypodermis. Options suggesting 2% or less than 2% significantly underestimate the skin's mass, as even the epidermis alone, though thin, covers a vast surface area (about 1.5-2 square meters in adults). A 10% estimate overshoots the typical range, as it would imply an unrealistically heavy skin mass for most individuals. The 7% figure is widely accepted in medical and biological contexts, reflecting the skin's substantial role in protection, thermoregulation, and sensation, and accounting for its multiple layers and associated structures like glands and hair.

Correct Answer: D

Rationale: Sebaceous glands are integral to skin function, producing sebum, an oily substance. They do not secrete sebum directly to the skin's surface; instead, they release it into hair follicles, which then reach the surface, making the first option inaccurate. They derive from the epidermis (ectoderm), not mesoderm, which forms the dermis, ruling out the second. They are exocrine glands, secreting sebum externally via ducts, not endocrine glands, which release hormones into the blood, eliminating the third. The correct description is that they are compound saccular (or alveolar) glands, with multiple acini (sac-like structures) branching from a duct, a histological classification fitting their structure and holocrine secretion mode (cells disintegrate to release sebum). This aligns with their role in lubricating skin and hair, their epidermal origin, and their anatomical complexity, distinguishing them from simpler gland types.

Correct Answer: D

Rationale: The hair matrix is the actively growing part of the hair bulb, located at the base of the hair follicle, and it contains a layer of basal cells (stem cells) that divide to produce the hair shaft and inner root sheath. The hair follicle is the entire structure surrounding the hair, not a component of the matrix. The hair shaft is the visible, dead portion of the hair, formed by the matrix but not part of it. The glassy membrane is a basement membrane-like structure around the follicle, not within the matrix. The basal cells in the matrix are critical for hair growth, as they differentiate and keratinize to form the hair, making this the accurate description.

Correct Answer: C

Rationale: Langerhans cells, found mainly in the stratum spinosum, are dendritic immune cells that detect and engulf bacteria, presenting antigens to trigger an immune response. Merkel cells sense touch, not infection. Keratinocytes form the skin barrier and produce keratin, but they don't directly fight bacteria. Melanocytes produce melanin for pigment, not immunity. When bacteria breach a wound, Langerhans cells act as the epidermis's first line of immune defense, migrating to lymph nodes to alert the immune system, making them the critical cells in this scenario.

Correct Answer: C

Rationale: Hair's waterproofing property stems from keratin, a tough, fibrous protein produced by keratinocytes in the hair follicle. Keratin forms the hair shaft's structure, creating a hard, insoluble layer that repels water and protects against environmental damage. This protein's high sulfur content, from cysteine amino acids, forms disulfide bonds, enhancing its durability and water resistance. Collagen, a major protein in the dermis, provides strength and support to skin but does not waterproof hair, as it's not a component of the hair shaft. Elastin, also in the dermis, offers flexibility, not waterproofing, and is absent in hair. Carotene is a pigment contributing to skin and hair color, not a protein or waterproofing agent. Keratin's role extends beyond hair to the epidermis, where it similarly prevents water loss, but in hair, its dense packing in the cuticle layer ensures a hydrophobic surface. This adaptation is critical for mammals, protecting hair from moisture-related degradation, making keratin the clear answer.