Which vitamin is essential for liver synthesis of clotting factors?

• A. Vitamin A
• B. Vitamin K
• C. Vitamin D
• D. Vitamin C

Answer: (B)

The key idea is that vitamin K acts as a necessary cofactor for a specific modification of several clotting factors produced by the liver. This modification, gamma-carboxylation, adds carboxyl groups to glutamate residues in factors II, VII, IX, and X (as well as proteins C and S), enabling these proteins to bind calcium and properly participate in the coagulation cascade. Without sufficient active vitamin K, these factors can’t be activated effectively, leading to impaired clotting and a longer clotting time. This is why vitamin K is essential for the liver’s production of functioning clotting factors. The other vitamins don’t play that specific role in gamma-carboxylation or the direct production of clotting factors.