Which vitamin is essential for clotting factor synthesis in the liver?

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

Answer: (C)

The concept being tested is why a specific vitamin is required for producing functional clotting proteins in the liver. Vitamin K acts as a key cofactor for the enzyme gamma-glutamyl carboxylase, which carboxylates certain glutamate residues on clotting factors II (prothrombin), VII, IX, and X, as well as proteins C and S. This gamma-carboxylation enables these proteins to bind calcium and phospholipid surfaces, which is essential for their proper function in the coagulation cascade. When these factors are properly carboxylated, they participate effectively in clot formation; without enough vitamin K, they remain undercarboxylated and inactive, leading to impaired clotting and a tendency to bleed.

Vitamin A is mainly involved in vision and epithelial health, not clotting factor synthesis. Vitamin D regulates calcium and phosphate metabolism and bone health. Vitamin C supports collagen synthesis and wound healing. So, while these vitamins have important roles, they do not directly enable the liver to synthesize the functional clotting factors that depend on vitamin K–mediated carboxylation.