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After 20 hours of fasting, supplies of glycogen will be exhausted, and in this situation, gluconeogenesis will play an important role. Since problems appeared only after glycogen was depleted, it must be a problem in gluconeogenesis, and not glycogen utilization. The glucose to lactate conversion was normal; therefore, it must be one of the four reactions peculiar to gluconeogenesis that can be defective. Tests for the PEPCK and pyruvate carboxylase show normal. From the glucogaon test, we come to know that glycogen degradation is normal; therefore, glucose-6-phosphatase must be normal (and this must be normal for glycogen utilization to happen during first 20 hours).
From these observations, we can conclude that the Fructose-1,6-bis-phosphatase is the most likely defective enzyme.
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