A patient with McArdle disease (GSD Type V, muscle phosphorylase deficiency) experiences painful cramps during exercise with no rise in venous lactate after forearm exercise test. Which feature best distinguishes this from GSD Type III (Cori disease, debranching enzyme deficiency)?

• A McArdle shows elevated liver enzymes; GSD III shows normal liver function
• B McArdle presents with cardiomegaly; GSD III does not
• C Both conditions affect only skeletal muscle with identical presentations
• D McArdle affects only muscle; GSD III affects both liver and muscle with hepatomegaly and fasting hypoglycemia ✓

Explanation

Muscle phosphorylase (McArdle, GSD V) is expressed only in skeletal muscle; liver phosphorylase is separate. Hence patients have no hepatomegaly, normal fasting glucose, but exercise intolerance. GSD III (Cori disease) involves the debranching enzyme (amylo-1,6-glucosidase) which is expressed in both liver and muscle — causing hepatomegaly, fasting hypoglycemia, AND myopathy. The ischemic forearm exercise test shows no lactate rise in both, but the systemic metabolic differences distinguish them.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

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Written and medically reviewed by the StethoPrep medical team.