What deficiency is indicated by a patient showing fatigue and muscle pain after exercise with a biopsy positive for periodic acid-Schiff staining?

The symptoms of fatigue and muscle pain after exercise, combined with a biopsy that is positive for periodic acid-Schiff (PAS) staining, indicate a glycogen storage disease characterized by impaired glycogen breakdown. A positive PAS stain suggests that there is an accumulation of glycogen in the tissues, which is consistent with glycogen storage diseases.

In the context of the answer being glycogen phosphorylase deficiency, this condition, also known as McArdle's disease, results in a deficiency of the enzyme responsible for breaking down glycogen into glucose. As a result, patients experience muscle pain and fatigue during physical activity, as their muscles cannot adequately access energy stored in glycogen due to the enzyme's unavailability. The biopsy highlights the accumulation of glycogen due to this inability to mobilize it effectively, leading to the mentioned symptoms.

Overall, the connection between exercise-induced symptoms and the findings from the biopsy aligns with glycogen phosphorylase deficiency, making it the most accurate conclusion based on the presented details.