Biochemistry

To solve the problem, we need to identify which of the listed nitrogenous substances is highly toxic.

1. Understanding the Toxicity of Nitrogenous Substances:
Ammonia, urea, uric acid, and hippuric acid are all nitrogenous waste products produced by organisms. Among them, ammonia is the most toxic substance. It is highly toxic because it requires a large amount of water to be safely excreted and can disrupt cellular functions at low concentrations.

2. Analyzing the Options:
Let's evaluate the toxicity of each substance:

• Ammonia: Ammonia is highly toxic to most organisms and is excreted mainly by aquatic animals, which can dilute it in water.
• Urea: Urea is less toxic than ammonia and is excreted by mammals. It requires less water for excretion.
• Uric acid: Uric acid is less toxic than ammonia and is excreted by birds and reptiles as a paste to conserve water.
• Hippuric acid: Hippuric acid is a waste product excreted by humans and is not highly toxic.

3. Conclusion:
The most toxic nitrogenous substance among the options is ammonia.

Final Answer:
The correct answer is (A) Ammonia.