Fat-soluble vitamins and hemostasis: Which fat-soluble vitamin plays the key role in regulating blood clotting through gamma-carboxylation of clotting factors?

Introduction / Context:
Hemostasis depends on a cascade of serine proteases whose activity requires post-translational modification. Vitamin K is essential for gamma-carboxylation of glutamate residues in several clotting factors, enabling calcium-dependent binding to phospholipid surfaces and proper coagulation.

Given Data / Assumptions:

• The question asks which vitamin regulates blood clotting.
• Fat-soluble vitamins include A, D, E, and K; only K is directly tied to clotting factor maturation.
• Niacin and vitamin C are water-soluble and have other roles.

Concept / Approach:
Vitamin K serves as a cofactor for the gamma-glutamyl carboxylase enzyme that activates clotting factors II, VII, IX, X and proteins C and S. In the absence of vitamin K, non-carboxylated proteins (PIVKAs) are produced, leading to bleeding diathesis. Antagonists such as warfarin inhibit vitamin K epoxide reductase, lowering active vitamin K and prolonging clotting times.

Step-by-Step Solution:
Identify vitamin–function mapping: vitamin K → clotting factor maturation.Rule out vitamins A and E (vision/antioxidant) and niacin (NAD+/NADP+ precursor).Select vitamin K as the correct answer.

Verification / Alternative check:
Clinical practice uses vitamin K to reverse warfarin over-anticoagulation; deficiency states show elevated PT/INR, supporting the central role in coagulation.

Why Other Options Are Wrong:

• Vitamin A: Vision, epithelial health, not coagulation.
• Vitamin C: Collagen and antioxidant roles; not clotting factor carboxylation.
• Niacin: Electron carrier synthesis; unrelated to clotting.
• Vitamin E: Antioxidant; can even increase bleeding risk at high doses.

Common Pitfalls:
Confusing clotting regulation (vitamin K) with platelet function or collagen-related vascular integrity (vitamin C).

Final Answer:
vitamin K.