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
First substrate of Kreb's cycle is: 
Select and explain. 
A. Pyruvate, B. Glycine, C. Citrate, D. Acetyl CoA
A: Pyruvate
Answer: A: Pyruvate 
Explanation: 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.