On exercise testing, a patient with McArdle disease exhibit:

McArdle's disease The Muscle phosphorylase enzyme is deficient, so glycogen cannot be conveed to glucose and fuher to lactate in exercising muscle. Hence, Normal or relatively low levels of lactate will be formed from muscle glycogen. Normal or low blood lactate and | blood ammonia after an ischemic exercise test suggest a defect in the conversion of glycogen (glucose) to lactate. Definitive diagnosis enzymatic assay in muscle tissue mutation analysis of the myophosphorylase gene. Extra Information: Abnormal exercise response also occurs with deficiencies of muscle phosphofructokinase deficiencies of debranching enzyme (when the test is done after fasting).