A child who was normal at bih develops chronic liver failure and muscle weakness at 3 months of age. On investigations, serum glucose is low, along with ketoacidosis and decreased pH. ALT and AST are raised. Blood lactate and uric acid levels are normal. Intravenous glucagon given after meals raises the blood glucose levels, but does not raise glucose when given after an overnight fast. Liver biopsy shows increased glycogen in liver. Which is the enzyme likely to be defective in this child?