Correct Answer:
B. Lactic acid
Muscle fatigue, particularly during intense exercise, is primarily attributed to the accumulation of lactic acid. When muscles work anaerobically (without sufficient oxygen), glucose is broken down into pyruvate, which is then converted to lactate (lactic acid). This buildup lowers the pH within muscle cells, interfering with muscle contraction mechanisms and leading to the sensation of fatigue and soreness.
- Option A, "Carbon dioxide," is a waste product of aerobic respiration, but its accumulation primarily affects blood pH and respiratory drive, not directly causing muscle fatigue in the same way as lactic acid.
- Option C, "Alcohol," is a metabolic byproduct not typically produced in significant amounts by muscle cells during exercise.
- Option D, "Urea," is a waste product of protein metabolism, excreted by the kidneys, and not directly linked to acute muscle fatigue.