A 24 year old graduate student presents to a physician with complaints of severe muscle cramps and weakness with even mild exercise. Muscle biopsy demonstrates glycogen accumulation, but hepatic biopsy is unremarkable. Which of the following is the most likely diagnosis?

A variety of glycogen storage diseases exist, corresponding to defects in different enzymes in glycogen metabolism; most of these involve the liver. McArdle's disease (Type V glycogen storage disease), due to a defect in muscle phosphorylase, is restricted to skeletal muscle. The presentation described in the question stem is typical. Many affected individuals also experience myoglobinuria. Definitive diagnosis is based on demonstration of myophosphorylase deficiency. Hanup's disease is a disorder of amino acid transpo. Krabbe's disease is a lysosomal storage disease. Niemann-Pick disease is a lysosomal storage disease. Ref: Cross H. (2012). Appendix I. Biochemical Basis of Diseases. In L.W. Janson, M.E. Tischler (Eds), The Big Picture: Medical Biochemistry.