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
Vit K is needed for which of these posttranslational modification processes 
Select and explain. 
A. Methylation, B. Carboxylation, C. Hydroxylation, D. Transketolation
B: Carboxylation
Answer: B: Carboxylation 
Explanation: Vitamin K functions as a cofactor for an enzyme carboxylase that catalyzes post-translation modification by carboxylation of glutamic acid (glutamate) residues on vitamin K dependent proteins.