A person has exercise intolerance, high muscle glycogen, and little or no blood lactate after exercise. Which deficiency best fits this pattern?

### Why this is right This is the characteristic pattern of McArdle syndrome, type V glycogenosis. Muscle glycogen phosphorylase deficiency prevents normal mobilization of muscle glycogen during exercise, limiting substrate supply for glycolysis and lactate production. ### The concept The stored glycogen is abundant but metabolically difficult to access. This explains why a storage disorder can produce high tissue glycogen alongside impaired exercise performance. ### The other options - **Liver glucose-6-phosphatase deficiency**: This causes von Gierke disease and disrupts blood-glucose release. - **Lysosomal alpha-1,4-glucosidase deficiency**: This causes Pompe disease and lysosomal glycogen accumulation. - **Liver glycogen phosphorylase deficiency**: This is Hers disease, involving hepatic glycogen breakdown. ### Terms - **McArdle syndrome**: Muscle glycogen phosphorylase deficiency. - **Glycolysis**: The pathway that breaks down glucose to pyruvate or lactate. ### Remember Exercise symptoms plus high muscle glycogen point to impaired muscle glycogen mobilization.