This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: ETC, Subject name: Biochemistry
Cyanide taken up by child. First one to be affected in Kreb's cycle is? 
Select and explain. 
A. Aconitase, B. NAD, C. Citrate, D. Acetyl CoA
B: NAD
Answer: B: NAD 
Explanation: Cyanide inhibits complex IV of ETC. So, if ETC is inhibited and NADH accumulates. If NADH accumulates, it means NAD depleted. So, TCA is affected because each TCA cycle requires 3 NAD (for Isocitrate Dehydrogenase, alpha-ketoglutarate dehydrogenase and Malate Dehydrogenase). Extra information: Cyanide poisoning: Cyanide is probably the most potent inhibitor of ETC (Complex IV). It binds to Fe3+ of cytochrome oxidase blocking mitochondrial respiration leading to cell death. Cyanide poisoning causes death due to tissue asphyxia (mostly of the central nervous system).