What is the most likely factor deficiency based on the following data? PT Normal, aPTT Prolonged, TT Normal

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

Answer: (C)

The pattern tests the different coagulation pathways. A normal PT indicates the extrinsic pathway is functioning, while a prolonged aPTT points to a problem in the intrinsic pathway or the common pathway. A normal thrombin time means the final step of converting fibrinogen to fibrin by thrombin is intact, so the issue is before thrombin acts, not with the conversion of fibrinogen or with crosslinking.

Among the intrinsic pathway factors listed, a deficiency in Factor IX specifically impairs the intrinsic pathway enough to prolong the aPTT, but it does not affect the extrinsic pathway (normal PT) or the final thrombin-mediated conversion of fibrinogen to fibrin (normal TT). This pattern fits Factor IX deficiency (hemophilia B) best.

Deficiencies in FII would typically prolong thrombin time because thrombin generation is impaired, and FXIII deficiency would not prolong the aPTT at all. FVII deficiency would prolong PT, not aPTT.