Which of the following is the most toxic nitrogenous waste?

Options

A. Carbon dioxide
B. Urea
C. Uric acid
D. Ammonia

Answer

### Step-by-Step Solution:

1. **Identify the Types of Nitrogenous Wastes**:
- There are three main types of nitrogenous wastes produced by organisms: ammonia, urea, and uric acid.

2. **Understand Toxicity Levels**:
- Among these nitrogenous wastes, ammonia is known to be the most toxic. Urea is less toxic than ammonia, and uric acid is the least toxic of the three.

3. **Excretion Methods**:
- Organisms that excrete ammonia are called ammonotelic, those that excrete urea are called ureotelic, and those that excrete uric acid are called uricotelic.

4. **Formation of Ammonia**:
- Ammonia is formed through the deamination of amino acids, where the amino group is removed, resulting in the production of ammonia (NH3).

5. **Water Requirement for Excretion**:
- Ammonia requires a large amount of water for its excretion; for instance, approximately 500 ml of water is needed to eliminate just 1 gram of ammonia. This is why only aquatic organisms can afford to excrete ammonia directly.

6. **Conclusion**:
- Based on the toxicity levels and the need for water for excretion, ammonia is identified as the most toxic nitrogenous waste among the options provided.

### Final Answer:
The most toxic nitrogenous waste is **ammonia**.

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