The part of mammalian skin that excretes metabolic wastes is?

Step 1: Analyze the question The question asks which part of mammalian skin excretes metabolic wastes. Excretion means removing waste products of metabolism, such as water, urea, and salts, from the body.

Step 2: Explain the concept The mammalian skin has several parts, each with its own function:

• Sweat gland: Produces sweat, which is mostly water with dissolved urea and mineral salts (mainly sodium chloride). It removes metabolic wastes and also helps cool the body.
• Horny layer (stratum corneum): The outer layer of dead, keratinized cells. It protects against water loss, germs, and injury, but it excretes nothing.
• Malpighian layer: The deepest layer of the epidermis, where new cells are made by cell division. It also contains melanin, which protects against UV rays.
• Sebaceous gland: Secretes sebum (oil), which lubricates the skin and hair and keeps them waterproof. Sebum is a secretion, not a metabolic waste.

Step 3: Work through the options

• A. Sweat gland: excretes water, urea, and salts, so it fits.
• B. Horny layer: protective only, so it does not fit.
• C. Malpighian layer: produces new cells and pigment, so it does not fit.
• D. Sebaceous gland: secretes oil for lubrication, so it does not fit.

Step 4: Answer The correct answer is A. sweat gland.

Exam tip: Remember the difference between excretion and secretion. Sweat glands excrete waste, while sebaceous glands secrete a useful oil. A quick memory aid is "Sweat = Waste out, Sebum = Smooth skin."