The major constituent of horns, nails, hairs and feathers of birds is-

Correct Answer: B

Rationale: The correct answer is B: Keratin. Keratin is a fibrous structural protein found in the horns, nails, hairs, and feathers of birds. It provides strength and flexibility to these structures. Collagen (A) is a different protein found in connective tissues. Mucin (C) is a glycoprotein found in mucus. Cuticle (D) is a protective outer layer found in plants and insects, not in the mentioned structures. Therefore, based on the composition and function of horns, nails, hairs, and feathers, keratin is the most appropriate choice.

Correct Answer: B

Rationale: The correct answer is B: Stratum lucidum. This layer of the epidermis is characterized by its clear, transparent cytoplasm due to the presence of eleidin, a protein that replaces the keratohyalin in the granules of the stratum granulosum. Stratum malpighi (A) refers to the basal layer, stratum granulosum (C) contains granules with keratohyalin, and stratum corneum (D) is the outermost layer composed of dead keratinocytes. The transparency of cytoplasm is a unique feature of the stratum lucidum, making it the correct choice.

Correct Answer: A

Rationale: The correct answer is A because merocrine sweat glands produce odorless sweat. The body odor associated with sweating is caused by apocrine sweat glands, not merocrine glands. Merocrine sweat helps prevent bacterial growth (B) through its antimicrobial properties. It contains water and waste products (C) such as urea and salts. Merocrine sweat is mostly water (D), aiding in cooling the body through evaporation. Therefore, A is incorrect as merocrine sweat does not produce body odor.

Correct Answer: B

Rationale: The correct answer is B: Blood flow to the skin increases. When body temperature rises, blood vessels in the skin dilate to release heat through radiation. This increased blood flow helps cool the body. Melanocytes (A) are responsible for skin pigmentation and do not directly regulate body temperature. Circulation in the skin (C) actually increases to dissipate heat. Evaporative cooling (D) through sweat continues to help regulate body temperature.

Correct Answer: C

Rationale: The correct answer is C: skin oil. Sebum is a waxy, oily substance produced by the sebaceous glands in the skin. It helps to lubricate and waterproof the skin and hair. Regular sweat (option A) is produced by sweat glands and is primarily composed of water and electrolytes. Milk (option B) is produced by mammary glands in mammals. Odiferous sweat (option D) refers to sweat that may have a strong odor due to bacterial activity, not sebum. Therefore, the correct term for the substance produced by the sebaceous glands is skin oil, making option C the correct answer.