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
3 ATP's are formed in the following steps of kreb's cyclea) Isocitrate dehydrogenaseb) Succinate dehydrogenasec) Succinate thiokinased) Malate dehydrogenase 
Select and explain. 
A. b, B. ad, C. ac, D. ab
B: ad
Answer: B: ad 
Explanation: Isocitrate dehydrogenase & Malate dehydrogenase

According to older calculations, 3 ATPs (via NADH) are formed at : (i) Isocitrate dehydrogenase, (iii) a-ketoglutarate dehydrogenase, and (iii) Malate dehydrogenase.
However, according to recent studies only 2.5 ATPs are produced (via NADH) at all these steps.