This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: TCA Cycle, Subject name: Biochemistry
[{'content': "First substrate of Kreb's cycle is: \nSelect and explain. \nA. Pyruvate, B. Glycine, C. Citrate, D. Acetyl CoA", 'role': 'user'}, {'content': 'Answer: A \nExplanation: First Substrate of TCA cycle Best answer--Oxaloacetate Second best--pyruvate (because it can form Oxaloacetate by enzyme pyruvate Carboxylase). If these not given, then go for acetyl CoA.', 'role': 'assistant'}]