Accumulation of which of the following molecules in the muscles occurs as a result of vigorous exercise ?

Options

A. Glycogen
B. Glucose
C. Pyruvic acid
D. L-lactic acid

Explanation

**Step-by-Step Solution:**

1. **Understanding Energy Production in Muscles:**
- The primary source of energy for muscle contraction during exercise is glucose.
- Glucose is stored in the body as glycogen, which can be converted back to glucose when energy is needed.

2. **Conversion of Glucose:**
- When glucose is metabolized, it is first converted into pyruvic acid through a process called glycolysis.
- This process typically occurs in the presence of oxygen, leading to the production of energy, carbon dioxide (CO2), and water (H2O).

3. **Vigorous Exercise and Oxygen Availability:**
- During vigorous exercise, the demand for energy increases significantly.
- If the oxygen supply is insufficient (due to high intensity), the body cannot fully oxidize pyruvic acid.

4. **Formation of Lactic Acid:**
- In the absence of sufficient oxygen, pyruvic acid is converted into lactic acid instead of being fully oxidized.
- This process is known as anaerobic respiration.

5. **Accumulation of Lactic Acid:**
- Lactic acid accumulates in the muscles as a result of this anaerobic metabolism during intense exercise.
- The accumulation of lactic acid can lead to muscle fatigue and discomfort.

6. **Conclusion:**
- Therefore, the correct answer to the question regarding which molecule accumulates in the muscles as a result of vigorous exercise is **lactic acid**.

**Final Answer: Lactic Acid (Option 4)**

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