The excretory product of some reptiles, birds and insects is

1. Analyze the question The question asks for the main nitrogenous waste excreted by some reptiles, birds and insects.

2. Explain the concept Animals break down proteins and amino acids, which produces nitrogenous waste. The form of waste depends on how much water the animal can afford to lose:

• Ammonia is very toxic and needs a lot of water to dilute it. Aquatic animals such as bony fish and tadpoles excrete it.
• Urea is less toxic and needs moderate water. Mammals (including humans) and amphibians excrete it.
• Uric acid is almost insoluble in water and non-toxic. It is excreted as a semi-solid paste with very little water loss, so it suits animals that must conserve water. Birds, many reptiles (lizards, snakes) and insects excrete it.

3. Step-by-step solution

• Reptiles, birds and insects live mostly on land, and their eggs are laid with shells or hard coverings. The embryo cannot get rid of waste, so a harmless, insoluble waste is needed.
• Water conservation is therefore important, which points to uric acid.
• Checking the options: • A. Urea is for mammals and amphibians. ✗ • B. Urine is a fluid containing waste, not a specific excretory chemical. ✗ • C. Ammonia is for aquatic animals. ✗ • D. Uric acid is for birds, reptiles and insects. ✓

4. Answer The correct option is D. uric acid.

5. Exam tip Remember "Birds, Reptiles, Insects = Uric acid", the water-savers. Link the waste to habitat: water animals excrete ammonia, mammals excrete urea, and dry-land egg-layers excrete uric acid.