In which condition would you expect to find defective glycogen breakdown in muscle?

McArdle's disease is characterized by a defect in the enzyme myophosphorylase, which is responsible for glycogen breakdown in muscle tissues. This impairment leads to an inability to adequately mobilize glucose from glycogen during periods of strenuous exercise. Individuals with McArdle's disease often experience muscle cramps, pain, and fatigue upon exertion due to the accumulation of glycogen in muscle fibers and the subsequent energy deficit.

In contrast, Pompe's disease primarily affects the heart and skeletal muscles due to the deficiency of acid alpha-glucosidase, causing an accumulation of glycogen in lysosomes. Andersen's disease involves a defect in branching enzyme leading to abnormal glycogen structure, while Cori's disease is caused by a deficiency in debranching enzyme, also affecting glycogen metabolism but resulting in milder symptoms compared to McArdle's.

Thus, McArdle's disease stands out as the condition where defective glycogen breakdown specifically manifests in the muscle, making it the correct answer.