Which coagulation test is most directly affected by the administration of vitamin K?

• A. Fibrinogen level
• B. Prothrombin time
• C. Platelet Count
• D. D-dimer

Answer: (B)

Vitamin K is essential for activating several coagulation factors, specifically II (prothrombin), VII, IX, and X, through gamma-carboxylation. Prothrombin time measures the time it takes for blood to clot via the extrinsic and common pathways, which depend on these vitamin K–dependent factors. Because factor VII, with the shortest half-life, is heavily involved in the extrinsic pathway, vitamin K deficiency or deficiency reversal most directly changes the PT. Replenishing vitamin K allows the liver to produce functional II, VII, IX, and X, normalizing the PT. Fibrinogen, platelets, and D-dimer are not directly driven by vitamin K status—their levels reflect different aspects of coagulation and fibrinolysis—so they aren’t as directly affected by vitamin K administration.