Which waste product is generally excreted by diffusion across body surface in ionic form?

Options

A. Urea
B. Ammonia
C. Uric acid
D. All of these

Answer

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks for the waste product that is excreted by diffusion across the body surface in ionic form.

2. **Identifying Waste Products**: The options provided are urea, ammonia, uric acid, and all of these. We need to analyze each of these waste products to determine which one is excreted in the manner described.

3. **Analyzing Ammonia**:
- Ammonia is a waste product that is highly soluble in water.
- It is primarily excreted by aquatic organisms, such as bony fishes, aquatic amphibians (like tadpoles), and some aquatic insects.
- These organisms excrete ammonia directly into the surrounding water, where it diffuses across their body surface or through gills.

4. **Examining Urea and Uric Acid**:
- Urea is less toxic than ammonia and is excreted by mammals and some amphibians. It is not typically excreted by diffusion but rather through the kidneys.
- Uric acid is even less soluble and is excreted as a paste by organisms like birds and reptiles, which conserves water. It is not excreted in ionic form.

5. **Conclusion**: Since ammonia is the only waste product that is excreted by diffusion across the body surface in ionic form (as ammonium ions), the correct answer to the question is ammonia.

### Final Answer:
**Ammonia** is the waste product that is generally excreted by diffusion across the body surface in ionic form.

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