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
Which step in TCA cycle is irreversible? 
Select and explain. 
A. Succinate thiokinase, B. Alpha ketoglutarate dehydrogenase, C. ISO citrate dehydrogenase, D. Aconitase
B: Alpha ketoglutarate dehydrogenase
Answer: B: Alpha ketoglutarate dehydrogenase 
Explanation: Ans. (b) Alpha ketoglutarate dehydrogenaseRef: Harper's Biochemistry 28/e, ch-9, p 306