D

After ~24 h of a zero-carbohydrate, high-protein diet, liver glycogen is exhausted, so maintenance of blood glucose depends almost entirely on hepatic gluconeogenesis from amino-acid carbon skeletons. If phosphoenol-pyruvate carboxykinase (“PEP carboxylase”) is inhibited, oxaloacetate cannot be converted to phosphoenol-pyruvate—the committed, rate-limiting step of gluconeogenesis—so glucose production falls and fasting hypoglycemia develops, as seen in inherited PEPCK deficiency presenting with profound fasting hypoglycemia [Hommes 1976, PMID 176867].