The first reaction of Kreb's cycle includes:

Options

A. Condensation of citric acid with acetyl CoA and formation off alpha- ketoglutaric acid
B. Condensation off cis-aconitic acid with acetyl coenzyme-A and formation of citric acid
C. Condensation of oxalosuccinic acid with acetyl CoA and formation of citric acid
D. Condensation of oxaloacetic acid with acetyl CoA and formation of citric acid

Answer

To solve the question regarding the first reaction of the Krebs cycle, we will analyze each option provided and determine which one correctly describes the initial reaction in the cycle.

### Step-by-Step Solution:

1. **Understanding the Krebs Cycle**:
- The Krebs cycle, also known as the citric acid cycle or TCA cycle, is a series of chemical reactions used by all aerobic organisms to generate energy through the oxidation of acetate derived from carbohydrates, fats, and proteins.

2. **Identifying the First Reaction**:
- The first reaction of the Krebs cycle involves the combination of a four-carbon molecule with a two-carbon molecule to form a six-carbon molecule.

3. **Analyzing the Options**:
- **Option 1**: "Condensation of citric acid with acetyl CoA and formation of alpha-ketoglutaric acid."
- This is incorrect because citric acid is not involved in the first reaction; it is formed later in the cycle.

- **Option 2**: "Condensation of cis-aconitic acid with acetyl CoA and formation of citric acid."
- This is incorrect because cis-aconitic acid is derived from citric acid and not involved in the first reaction.

- **Option 3**: "Condensation of oxalosuccinic acid with acetyl-CoA and formation of citric acid."
- This is incorrect because oxalosuccinic acid is not a reactant in the first step of the Krebs cycle.

- **Option 4**: "Condensation of oxaloacetic acid with acetyl-CoA and formation of citric acid."
- This is correct. In the first reaction, oxaloacetic acid (a four-carbon molecule) condenses with acetyl-CoA (a two-carbon molecule) to form citric acid (a six-carbon molecule).

4. **Conclusion**:
- The correct answer is **Option 4**: "Condensation of oxaloacetic acid with acetyl-CoA and formation of citric acid."