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 of the following cannot be phosphorylated using protein kinase 
Select and explain. 
A. Aspergine, B. Threonine, C. Serine, D. Tyrosine
A: Aspergine
Answer: A: Aspergine 
Explanation: Protein kinase mostly add phosphoryl group to a serine, threonine or tyrosine residues.