In which step of Krebs cycle, `CO_(2)` is evolved

Options

A. Isocitric acid `rarr`Oxalosuccinic acid
B. Succinic acid `rarr` Fumaric acid
C. Oxalosuccinic acid `rarr alpha`-ketoglutaric acid
D. Maleic acid `rarr` Oxaloacetic acid.

Answer

To determine in which step of the Krebs cycle carbon dioxide (CO₂) is evolved, we can analyze each option provided in the question.

### Step-by-Step Solution:

1. **Understanding the Krebs Cycle**: The Krebs cycle, also known as the citric acid cycle, is a series of chemical reactions used by all aerobic organisms to generate energy. During this cycle, carbon dioxide is released as a byproduct in certain steps.

2. **Identifying the Process of Decarboxylation**: The release of carbon dioxide in the Krebs cycle occurs during a process called decarboxylation. This is when a carboxyl group is removed from a molecule, releasing CO₂.

3. **Analyzing Each Option**:
- **Option 1**: Isocitric acid to oxalosuccinic acid.
- In this step, NADH and H⁺ ions are produced, but no carbon dioxide is released. Therefore, this option is incorrect.

- **Option 2**: Succinic acid to fumaric acid.
- In this step, FADH₂ is produced, but again, no carbon dioxide is released. Thus, this option is also incorrect.

- **Option 3**: Oxalosuccinic acid to alpha-ketoglutaric acid.
- This step involves the release of one molecule of carbon dioxide. Therefore, this is the correct answer.

- **Option 4**: Malic acid to oxaloacetic acid.
- In this conversion, NADH and H⁺ ions are produced, but no carbon dioxide is released. Hence, this option is incorrect.

4. **Conclusion**: The correct answer is **Option 3**, where carbon dioxide is evolved when oxalosuccinic acid converts to alpha-ketoglutaric acid.

### Final Answer:
**Carbon dioxide is evolved in the step where oxalosuccinic acid converts to alpha-ketoglutaric acid (Option 3).**