Glycogen storage disease type III (Cori disease) is caused by debranching enzyme (amylo-1,6-glucosidase/4-alpha-glucanotransferase) deficiency. Glycogen structure in Cori disease differs from that in McArdle disease (GSD V) in that:

• A Cori disease accumulates glucose-6-phosphate; McArdle disease accumulates glucose-1-phosphate
• B Cori disease causes hepatomegaly only; McArdle disease causes cardiomegaly only
• C Cori disease accumulates phosphorylase limit dextrin with short outer branch chains; McArdle disease accumulates normal glycogen structure with inability to mobilise it in muscle ✓
• D Both diseases accumulate identical glycogen structures but affect different tissues

Explanation

In Cori disease (GSD III), absent debranching enzyme means phosphorylase can degrade outer branches only until 4 glucose residues remain at each branch point (a phosphorylase limit dextrin). These shortened-branch glycogen molecules accumulate. In McArdle disease (GSD V), myophosphorylase is absent in skeletal muscle. Normal glycogen is synthesised correctly (by glycogen synthase and branching enzyme), but cannot be mobilised during exercise, causing exercise-induced myalgia, cramps, and myoglobinuria. The glycogen structure in McArdle disease is architecturally normal—just inaccessible. The distinction: Cori = abnormal glycogen structure (truncated branches); McArdle = normal glycogen structure but mobilisation failure.

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

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Written and medically reviewed by the StethoPrep medical team.