This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Link reaction, Subject name: Biochemistry
All of the following are tightly bound to component enzymes of pyruvate dehydrogenase, EXCEPT 
Select and explain. 
A. Thiamine pyrophosphate, B. Lipoic acid, C. Flavin adenine dinucleotide, D. Coenzyme A
D: Coenzyme A
Answer: D: Coenzyme A 
Explanation: None