This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Kerbs Cycle, Subject name: Biochemistry
[{'content': 'Which step in TCA cycle is irreversible? \nSelect and explain. \nA. Succinate thiokinase, B. Alpha ketoglutarate dehydrogenase, C. ISO citrate dehydrogenase, D. Aconitase', 'role': 'user'}, {'content': "Answer: B \nExplanation: Ans. (b) Alpha ketoglutarate dehydrogenaseRef: Harper's Biochemistry 28/e, ch-9, p 306", 'role': 'assistant'}]