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 reactions does 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 of hydrolyzing 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'}]