How many of the following are the substrates involved in the Kreb's cycle?
Citric acid, succinic acid, hydrochloric acid, a-ketoglutaric acid, fumaric acid, malic acid, pyruvic acid, phosphoglyceric acid, acetic acid

Options

A. six
B. four
C. five
D. three

Answer

To determine how many of the given substances are substrates involved in the Kreb's cycle, we will analyze each substance listed in the question.

### Step-by-Step Solution:

1. **List the Given Substances**:
- Citric acid
- Succinic acid
- Hydrochloric acid
- Alpha-ketoglutaric acid
- Fumaric acid
- Malic acid
- Pyruvic acid
- Phosphoglyceric acid
- Acetic acid

2. **Identify Substrates in the Kreb's Cycle**:
- The Kreb's cycle (also known as the citric acid cycle or TCA cycle) involves several key substrates. Let's identify which of the listed substances are part of this cycle:
- **Citric acid**: Yes, it is the first substrate formed in the cycle.
- **Succinic acid**: Yes, it is formed during the cycle.
- **Hydrochloric acid**: No, it is not involved in the Kreb's cycle.
- **Alpha-ketoglutaric acid**: Yes, it is a substrate in the cycle.
- **Fumaric acid**: Yes, it is also a substrate in the cycle.
- **Malic acid**: Yes, it is formed in the cycle.
- **Pyruvic acid**: No, it is not directly part of the Kreb's cycle; it is converted to acetyl-CoA before entering the cycle.
- **Phosphoglyceric acid**: No, it is part of glycolysis, not the Kreb's cycle.
- **Acetic acid**: No, it is not a substrate in the Kreb's cycle.

3. **Count the Valid Substrates**:
- From the analysis, the valid substrates involved in the Kreb's cycle are:
- Citric acid
- Succinic acid
- Alpha-ketoglutaric acid
- Fumaric acid
- Malic acid
- Total valid substrates = 5

4. **Conclusion**:
- The number of substrates from the given list that are involved in the Kreb's cycle is **5**.

### Final Answer:
The correct answer is option **C: 5**.