Baby has hypoglycaemia, specially early morning hypoglycaemia. Glucagon given. It raises blood glucose if given after meals But does not raises blood glucose if given during fasting. Liver biopsy shows increased glycogen deposits. Enzyme defect is ?

This patient has Debranching enzyme deficiency, also known as limit dextrinosis or Cori's Disease. Normally this enzyme releases just 10% glucose. So the patient has only early morning hypoglycemia in its deficiency i.e. mild hypoglycemia. (No hypoglycemia during the daytime, when meals are taken). Limit dextrins (a branched oligosaccharide having lots of alpha 1--> 6 bonds) are accumulated in this patient's liver during fasting so if glucagon given during fasting, phosphorylase (rate-limiting enzyme of glycogenolysis, which is activated by Glucagon) will be activated which cannot break limit dextrins. Phosphorylase enzyme can only break alpha 1--> 4 bonds. So this will not raise blood glucose. In fed state glycogen is formed. So if glucagon is given during fed state, phosphorylase will be activated which can break glycogen (has both alpha 1--> 4 & alpha 1-->6 glycosidic bonds) & can raise blood glucose. In von Gierke's disease, if glucagon is given, blood glucose will not be increased and in this disease, the patient has severe hypoglycemia as both glycogenolysis and gluconeogenesis cannot occur due to deficiency of Glucose-6-phosphatase - So, Option B is not the answer.