Clinical biochemistry: A deficiency of vitamin K leads most directly to decreased levels of which coagulation factor in plasma?

Introduction / Context:
Vitamin K is essential for hepatic γ-carboxylation of specific glutamate residues on several clotting factors, enabling calcium binding and normal coagulation. Identifying the most directly affected factor is a common exam point.

Given Data / Assumptions:

• Vitamin K–dependent factors include II (prothrombin), VII, IX, and X, plus proteins C and S.
• Carboxylation occurs in the liver, enabling interaction with phospholipid surfaces.
• Deficiency leads to prolonged bleeding and abnormal coagulation tests.

Concept / Approach:
Prothrombin (factor II) is the direct zymogen precursor of thrombin. Its synthesis and activation depend on vitamin K–mediated γ-carboxylation. Without vitamin K, prothrombin levels and activity decrease, impairing thrombin generation and fibrin formation.

Step-by-Step Solution:
Vitamin K deficiency → impaired γ-carboxylation in factors II, VII, IX, X.Among options, prothrombin (II) is the vitamin K–dependent zymogen listed.Therefore, decreased prothrombin is the most direct effect.

Verification / Alternative check:
Correction of coagulopathy with vitamin K administration in deficiency states demonstrates the dependence of prothrombin activation on vitamin K.

Why Other Options Are Wrong:

• Thrombin and fibrin are downstream products; their reduction is secondary.
• Fibrinogen (factor I) synthesis is not vitamin K–dependent.

Common Pitfalls:
Confusing synthesis (zymogens) with activated enzymes; vitamin K acts earlier at the zymogen modification stage.

Final Answer:
prothrombin