Which of the following enzymes is absent in mitochondria

Options

A. Aconitase
B. Malic dehydrogenase
C. Fumarase
D. Hexokinase.

Answer

### Step-by-Step Solution:

1. **Understanding the Question**: The question asks which enzyme is absent in mitochondria among the given options: aconitase, malic dehydrogenase, fumarase, and hexokinase.

2. **Analyzing Each Enzyme**:
- **Aconitase**: This enzyme catalyzes the conversion of citrate to cis-aconitate, a reaction that occurs in the Krebs cycle (also known as the citric acid cycle), which takes place in the mitochondria. Therefore, aconitase is present in mitochondria.

- **Malic Dehydrogenase**: This enzyme catalyzes the reaction from malate to oxaloacetate, which is also a part of the Krebs cycle. Since this reaction occurs in the mitochondria, malic dehydrogenase is present in mitochondria.

- **Fumarase**: Fumarase catalyzes the conversion of fumarate to malate, another reaction in the Krebs cycle. This reaction occurs in the mitochondria, indicating that fumarase is also present in mitochondria.

- **Hexokinase**: This enzyme catalyzes the conversion of glucose to glucose-6-phosphate, which is the first step in the glycolysis pathway. Glycolysis occurs in the cytoplasm, not in the mitochondria. Thus, hexokinase is absent in mitochondria.

3. **Conclusion**: Among the options provided, hexokinase is the only enzyme that is absent in mitochondria. Therefore, the correct answer is hexokinase.

### Final Answer:
**Hexokinase** is the enzyme that is absent in mitochondria.

---