Explain the role of the deficient enzyme.

Muscle glycogen phosphorylase is essential for breaking down glycogen stores in muscle cells into glucose-1-phosphate, which then enters glycolysis to produce energy.

During exercise, especially intense exercise, muscles rely heavily on glycogen breakdown for energy. Without functional muscle glycogen phosphorylase, glycogen cannot be broken down, meaning glucose cannot be supplied to the glycolytic pathway from this source.

Since glycolysis is impaired due to the lack of substrate from glycogen, there is minimal production of pyruvate. Pyruvate is the precursor to lactate, which is formed during anaerobic metabolism. Therefore, blood lactate levels do not rise significantly during exercise in individuals with McArdle disease because the primary pathway for lactate production from muscle glycogen is blocked.

Blood lactate does not rise in McArdle disease during exercise because the deficient enzyme, muscle glycogen phosphorylase, prevents the breakdown of muscle glycogen into glucose, which is necessary for glycolysis and subsequent lactate production.