Identify enzyme A in the given reaction of Krebs' cycle
`underset((4C))(OA A) +"Acety"1-CoA +H_(2)O overset(A)rarr underset((6C))("Citric acid") +CoA`

Options

A. Oxaloacetate synthetase
B. Citrate synthase
C. Aconitase
D. Dehydrogenase

Answer

### Step-by-Step Solution

1. **Understanding the Reaction**: The reaction involves a 4-carbon compound, oxaloacetic acid (OA), reacting with acetyl-CoA and water to produce a 6-carbon compound, citric acid, and CoA.

2. **Identifying the Components**:
- The 4-carbon compound is oxaloacetic acid (C4).
- The 2-carbon compound is acetyl-CoA (C2).
- The product formed is citric acid (C6), which is a key intermediate in the Krebs cycle.

3. **Recognizing the Enzyme's Role**: In the Krebs cycle, the enzyme that catalyzes the reaction between oxaloacetic acid and acetyl-CoA to form citric acid is crucial. This enzyme facilitates the condensation of these two substrates.

4. **Naming the Enzyme**: The enzyme that catalyzes this reaction is known as citrate synthase. It is responsible for the first step of the Krebs cycle, where citric acid is formed.

5. **Conclusion**: Therefore, the enzyme A in the given reaction of the Krebs cycle is citrate synthase.

### Final Answer
**Enzyme A is citrate synthase.**

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