If PT is prolonged with a normal aPTT, which factor deficiency is most likely?

• A. FVII
• B. FII
• C. FIX
• D. FXIII

Answer: (A)

Prothrombin time looks at the extrinsic pathway, which hinges on Factor VII, and the common pathway. A prolonged PT with a normal aPTT means the intrinsic pathway is intact, so the likely defect lies in the extrinsic pathway. A deficiency of Factor VII specifically disrupts the extrinsic pathway, causing PT to lengthen while aPTT remains normal because the intrinsic pathway factors (VIII, IX, XI, XII) are unaffected.

Others don’t fit this pattern: a deficiency of prothrombin (Factor II) would prolong both PT and aPTT since it’s part of the common pathway; a deficiency of Factor IX (intrinsic pathway) would prolong aPTT; and a deficiency of Factor XIII doesn’t alter PT or aPTT because it acts after clot formation rather than during the generation of the fibrin clot.