In suspected vitamin K deficiency, which lab pattern is most typical?

• A. Elevated D-dimer
• B. Prolonged aPTT with low fibrinogen
• C. Normal PT with normal fibrinogen and platelets
• D. Prolonged PT with normal fibrinogen and platelets

Answer: (D)

Vitamin K is required to activate several clotting factors (II, VII, IX, and X). When vitamin K is deficient, these factors have reduced activity, and the extrinsic pathway is affected first because factor VII—shortest half-life among them—drives the PT. This causes a prolonged prothrombin time while fibrinogen levels and platelet counts remain normal, since vitamin K deficiency doesn’t directly alter fibrinogen production or platelet number. D-dimer isn’t a specific feature of vitamin K deficiency; it reflects clot breakdown and is more typical of conditions like DIC, thrombosis, or fibrinolysis. The aPTT may become prolonged later as II, IX, and X activity declines, but the classic early pattern is prolonged PT with normal fibrinogen and platelets.