Vitamin K deficiency characteristically affects coagulation. Which laboratory parameter is most affected?

• A. Prolonged prothrombin time
• B. Prolonged partial thromboplastin time
• C. Prolonged bleeding time
• D. International normalized ratio

Answer: (D)

Vitamin K is required to activate several clotting factors (II, VII, IX, X). Without it, these factors are reduced in activity, impairing coagulation. The extrinsic pathway, assessed by prothrombin time, is particularly sensitive because factor VII has the shortest half-life, so PT becomes prolonged early in vitamin K deficiency. INR is the standardized version of PT, designed to allow comparison across different laboratories. When vitamin K deficiency lengthens PT, the INR rises accordingly, making it the most representative single parameter of the coagulation defect. Bleeding time stays normal since it reflects platelet function, not the vitamin K–dependent clotting factors, and aPTT may be affected later.