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
What is the substrate for the only physiologically irreversible reaction in the Kreb's Citric acid cycle? 
Select and explain. 
A. Citrate, B. Alpha ketoglutarate, C. Succinate, D. Malate
B: Alpha ketoglutarate
Answer: B: Alpha ketoglutarate 
Explanation: B i.e. Alphaketoglutarate