Muscle cells engaged in vigorous activity build up a high concentration of

Options

A. lactic acid
B. pyruvic acid
C. alcohol
D. cholesterol

Answer

### Step-by-Step Solution:

1. **Understanding Muscle Activity**: When muscle cells are engaged in vigorous activity, they require energy to sustain their function. This energy is primarily derived from the breakdown of carbohydrates.

2. **Energy Production Process**: Under normal circumstances, muscles utilize aerobic respiration, which requires oxygen. However, during intense exercise, the oxygen supply may not meet the demand, leading to a shift towards anaerobic respiration.

3. **Anaerobic Respiration**: In the absence of sufficient oxygen, muscle cells undergo anaerobic respiration. This process allows them to continue producing energy, but it results in the formation of lactic acid as a byproduct.

4. **Identifying the Byproduct**: The question specifically asks about the high concentration built up in muscle cells during vigorous activity. The key byproduct of anaerobic respiration in muscle cells is lactic acid.

5. **Evaluating Other Options**:
- **Pyruvic Acid**: This is produced during glycolysis but is not the main product during anaerobic respiration in muscle cells.
- **Alcohol**: This is produced during fermentation, not in muscle cells during exercise.
- **Cholesterol**: This is related to fat metabolism and is not directly involved in energy production during exercise.

6. **Conclusion**: Based on the understanding of muscle metabolism during vigorous activity, the correct answer is that muscle cells build up a high concentration of lactic acid.

### Final Answer:
Muscle cells engaged in vigorous activity build up a high concentration of **lactic acid**.

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