You are a doctor treating apatient who suffers from severe hypoglycemia after prolonged>24hr fasting. Glucose is converted to lactate normally in the patient's liver. You administer glucagon and find that bloodglucose increases normally. (The patient was in the well-fed stateprior to this test) The patients liver shows normal activitesof pyruvate carboxylase and PEP carboykinase. Which enzyme may showa genetic defect that accounts for these findings