Which of the following is NOT an excretory product of animals?

1. Analyze the question The question asks which option is NOT an excretory product of animals. Excretion is the removal of metabolic waste products from the body.

2. Explain the concept Excretion removes the waste made by chemical reactions (metabolism) inside the body. Common excretory products and the organs that remove them:

• Carbon dioxide: from respiration, removed by the lungs (or gills or skin)
• Urea: from the breakdown of excess amino acids, removed by the kidneys in urine
• Sweat: contains water, salts and a little urea, removed by the skin (sweat glands)
• Bile pigments: from the liver
• Oxygen: this is not a waste product in animals. It is a raw material taken in for respiration.

3. Step-by-step solution

• A. Carbon dioxide: a waste product of respiration, so it is excreted.
• B. Urea: a waste product of protein metabolism, so it is excreted.
• C. Sweat: it removes water, salts and urea, so it is excreted.
• D. Oxygen: animals take it in for respiration and do not excrete it. (In plants, oxygen is a by-product of photosynthesis, but the question is about animals.)

4. Answer The correct option is D. oxygen.

5. Exam tip Remember that in animals oxygen goes in, carbon dioxide comes out. If a question asks about animals, oxygen is a raw material, not a waste. Don't mix it up with plants, where oxygen is released during photosynthesis.