Lactic acid buildup during anaerobic exercise

Steps:

• During intense exercise, muscles use anaerobic respiration when oxygen is limited, producing lactic acid as a byproduct.
• This causes lactic acid concentration in blood to rise sharply during exercise.
• After exercise, as oxygen becomes available, the body clears lactic acid, so levels drop.
• Compare to other substances: only lactic acid fits this pattern of increase during and decrease after activity.

Why C is correct:

• Lactic acid forms via glycolysis in muscles under low-oxygen conditions (lactate dehydrogenase converts pyruvate to lactate), raising blood levels during exercise.

Why the others are wrong:

• A. Alcohol is not produced by human metabolism during exercise; it's from fermentation or ingestion.
• B. Glycogen is a stored form of glucose in muscles/liver, not typically measured as blood concentration fluctuating with exercise.
• D. Urea rises gradually from protein breakdown over days, not acutely during/after exercise.

Final answer: C