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: Biochemistry
Irreversible step in citric acid cycle is catalyzed by ? 
Select and explain. 
A. Aconitase, B. Succinate thiokinase, C. a-ketoglutarate dehydrogenase, D. Isocitrate dehydrogenase
C: a-ketoglutarate dehydrogenase
Answer: C: a-ketoglutarate dehydrogenase 
Explanation: Ans. is 'c' i.e., a-ketoglutarate dehydrogenase There are two irreversible steps :oxaloacetate to citrate catalysed by citrate (synthase), anda-ketoglutarate to succinyl CoA, catalyzed by a-ketoglutarate dehydrogenase.