Pigmentation of the skin is by the secretions of-

Correct Answer: C

Rationale: The correct answer is C: Melanocytes. Melanocytes are the cells responsible for producing melanin, the pigment that gives color to the skin. Monocytes, neutrophils, and eosinophils are types of white blood cells involved in immune responses and inflammation, not in pigmentation of the skin. Melanocytes specifically produce melanin, which determines the color of an individual's skin, hair, and eyes. Therefore, the secretion of melanocytes is responsible for skin pigmentation.

Correct Answer: D

Rationale: The correct answer is D: Sole. The thickest portion of skin in humans is found on the soles of the feet due to the constant pressure and friction they endure from walking and standing. This leads to a thicker layer of protective skin to provide support and cushioning. The head (A), thumb (B), and palm (C) do not experience the same level of pressure and friction as the soles, resulting in thinner skin layers in those areas. Thus, the sole is the correct choice for the thickest portion of skin in humans.

Correct Answer: A

Rationale: The correct answer is A: Simple tubular. Sweat glands are classified based on their shape and structure. Simple tubular glands have a long, straight tube for secretion, which matches the structure of sweat glands. Simple alveolar glands have a rounded sac shape, not seen in sweat glands. Compound acinar glands have a branched, sac-like structure, and compound tubular glands have a branched, tube-like structure, neither of which matches the structure of sweat glands. Therefore, the correct answer is A as sweat glands are characterized by a simple tubular structure.

Correct Answer: D

Rationale: The correct answer is D: Shrew. Shrews are known to have red-colored sweat due to a pigment called porphyrin. This pigment gives their sweat a reddish hue. Kangaroos, elephants, and rhinoceroses do not have red sweat. Kangaroos sweat is colorless, elephants' sweat is clear, and rhinoceroses sweat may appear red due to mud or dust, not because of any pigment. Therefore, the red-colored sweat is unique to shrews, making choice D the correct answer.

Correct Answer: D

Rationale: The correct answer is D: Cortex. Melanin pigment granules are found in the cortex layer of the hair shaft. This is where the color of the hair is determined. The cortex contains the majority of the hair's melanin, which gives hair its natural color. The other choices, A: Hair follicle, B: Hair papilla, and C: Medulla, do not contain melanin pigment granules. The hair follicle is the root of the hair, the hair papilla is a structure at the base of the follicle, and the medulla is the innermost layer of the hair shaft that may or may not contain pigment depending on the hair color.