The sensation of fatigue in the muscles after prolonged strenuous physical work is caused by

1. The question asks for the cause of muscle fatigue during prolonged strenuous physical work. 2. During intense exercise, muscle energy demand can exceed the oxygen supply available for aerobic respiration. 3. Muscles then rely more on anaerobic glycolysis to produce ATP, which breaks down glucose but also produces lactic acid as a byproduct. 4. The accumulation of lactic acid (and associated hydrogen ions) within the muscle cells lowers pH, inhibits enzyme function, interferes with calcium handling and muscle contraction, contributing significantly to the sensation of fatigue. 5. While oxygen decrease (A) triggers anaerobic metabolism and glucose depletion (C) eventually limits energy, lactic acid buildup (D) is the primary factor for acute fatigue during exertion.