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
Which one of the following TCA cycle intermediate may not be added or removed by other metabolic pathways? 
Select and explain. 
A. Oxaloacetate, B. Isocitrate, C. Citrate, D. Fumarate
B: Isocitrate
Answer: B: Isocitrate 
Explanation: Oxaloacetate may be formed from pyruvate during gluconeogenesis. Degradation of tyrosine and phenylalanine may produce fumarate. Citrate is transpoed out of mitochondria to be used as a source of cytoplasmic acetyl CoA. Isocitrate is formed and removed in TCA cycle. Ref: Textbook of Biochemistry for Medical Students by D.M. Vasudevan, 6th Edition, Page: 218