Which of the following is the only 5-carbon compound formed during Krebs cycle

Options

A. Malic acid
B. Succinic acid
C. cis-aconitic acid
D. `alpha`-ketoglutaric acid.

Answer

To solve the question "Which of the following is the only 5-carbon compound formed during the Krebs cycle?" we will analyze each of the given options based on their structure and the number of carbon atoms they contain.

### 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. It involves various intermediates, and we need to identify the 5-carbon compound among the options provided.

2. **Analyzing the Options**:
- **Malic Acid**: This is also known as L-malate. It is an intermediate in the Krebs cycle, but it contains **4 carbon atoms**. Therefore, it cannot be the correct answer.
- **Succinic Acid**: Also known as succinate, this compound is another intermediate in the Krebs cycle. It also contains **4 carbon atoms**. Thus, this option is incorrect as well.
- **Cis-Aconitic Acid**: This compound is formed in the Krebs cycle and has **6 carbon atoms**. Therefore, it is not the 5-carbon compound we are looking for.
- **Alpha-Ketoglutaric Acid**: This is a key intermediate in the Krebs cycle. It consists of **5 carbon atoms** (it has 2 carboxyl groups and 3 carbon chains). This is the only 5-carbon compound among the options.

3. **Conclusion**: Based on the analysis, the only 5-carbon compound formed during the Krebs cycle is **Alpha-Ketoglutaric Acid**.

### Final Answer:
**Alpha-Ketoglutaric Acid (Option D)** is the only 5-carbon compound formed during the Krebs cycle.

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