The correct option is Lactic acid.
Explanation
Cellular respiration is the biochemical process by which cells break down glucose to release energy. While this typically occurs in the presence of oxygen (aerobic respiration), muscle cells can switch to anaerobic respiration when oxygen supply is insufficient to meet the energy demand.
Analysis of the Process:
- Glycolysis: The first step of respiration involves breaking down glucose (a 6-carbon molecule) into pyruvate (a 3-carbon molecule) within the cytoplasm.
- Anaerobic Respiration in Muscles: During sudden physical activity, the oxygen supply to muscle cells may be inadequate. In this situation, pyruvate is converted into lactic acid (a 3-carbon molecule) and energy.
- Cause of Cramps: The rapid accumulation of lactic acid in the muscle tissues leads to acidity and fatigue, resulting in muscle cramps.
- Comparison with Other Options:
- Ethanol: This is produced during anaerobic respiration (fermentation) in yeast, along with carbon dioxide, not in human muscles.
- Acetone and Methane: These are not byproducts of glucose metabolism in human muscle cells.
Key Takeaway: Muscle cramps occur due to the accumulation of lactic acid, which is produced when muscle cells break down glucose anaerobically due to a lack of oxygen.