Hers disease is due to deficiency of which enzyme?

• A. Glycogen phosphorylase
• B. Glycogen branching enzyme
• C. Debranching enzyme
• D. Glucose-6-phosphatase

Answer: (A)

Hers disease tests your understanding that glycogen breakdown in the liver depends on glycogen phosphorylase. When this enzyme is deficient, the liver cannot effectively convert stored glycogen into glucose-1-phosphate for release as glucose during fasting, so the liver becomes enlarged and fasting glucose remains relatively low. This specific deficiency lines up with Hers disease (a hepatic glycogen storage disorder).

Other enzyme deficiencies cause different disorders: lacking the branching enzyme leads to abnormal glycogen structure (Andersen disease); lacking the debranching enzyme leads to Cori disease with impaired glycogen breakdown and dextrin-like accumulations; lacking glucose-6-phosphatase leads to Von Gierke disease with more severe hypoglycemia and metabolic complications.