This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Surgery
[{'content': 'Among the many molecules of high-energy phosphate compounds formed as a result of the functioning of the citric acid cycle, one molecule is synthesized at the substrate level. In which of the following reactionsdoes this occur? \nSelect and explain. \nA. Citrate - a-ketoglutarate, B. a-ketoglutarate - succinate, C. Succinate - fumarate, D. Fumarate - malate', 'role': 'user'}, {'content': 'Answer: B \nExplanation: A molecule of guanosine triphosphate is synthesized from guanosine diphosphate and phosphate at the cost ofhydrolyzing succinyl CoA to succinate and CoA. This constitutes substrate-level phosphorylation, and, in contrast to oxidative phosphorylation, this is the only reaction in the citric acid cycle that directly yields a high-energy phosphate bond. The sequence of reactions from alpha-ketoglutarate to succinate is catalyzed by the a -ketoglutarate dehydrogenase complex and succinyl-CoA synthetase, respectively.', 'role': 'assistant'}]