Which of the following are the 4C substrates for oxidation reactions of Krebs cycle?
I. a-ketoglutaric acid II Fumaric acid
III Malci acid IV Isocitric acid

Options

A. I, II
B. II, III
C. I, III
D. II, IV

Answer

To determine which of the given compounds are 4C substrates for oxidation reactions of the Krebs cycle, we need to analyze the molecular formulas of each compound provided in the question.

### Step-by-Step Solution:

1. **Identify the Compounds**: The compounds given are:
- I. Alpha-ketoglutaric acid
- II. Fumaric acid
- III. Malic acid
- IV. Isocitric acid

2. **Determine the Molecular Formulas**:
- **Alpha-ketoglutaric acid**: C5H6O5 (5 carbon atoms)
- **Fumaric acid**: C4H4O4 (4 carbon atoms)
- **Malic acid**: C4H6O5 (4 carbon atoms)
- **Isocitric acid**: C6H8O7 (6 carbon atoms)

3. **Count the Carbon Atoms**:
- Alpha-ketoglutaric acid has 5 carbon atoms (not a 4C substrate).
- Fumaric acid has 4 carbon atoms (is a 4C substrate).
- Malic acid has 4 carbon atoms (is a 4C substrate).
- Isocitric acid has 6 carbon atoms (not a 4C substrate).

4. **Identify the 4C Substrates**: From the analysis, the compounds that have 4 carbon atoms are:
- Fumaric acid (II)
- Malic acid (III)

5. **Conclusion**: The correct answer is that the 4C substrates for oxidation reactions of the Krebs cycle are:
- II. Fumaric acid
- III. Malic acid

### Final Answer:
The 4C substrates for oxidation reactions of the Krebs cycle are Fumaric acid (II) and Malic acid (III).

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