An 8-year-old boy with a lifelong history of mild bleeding has an isolated prolongation of prothrombin time; activated partial thromboplastin time, platelet count and bleeding time are all normal. Mixing study fully corrects. Which single factor deficiency explains this pattern?

• A Factor VII ✓
• B Factor XI
• C Factor VIII
• D Factor XIII

Explanation

The prothrombin time screens the extrinsic pathway plus the common pathway, while the aPTT screens the intrinsic pathway plus the common pathway. An isolated prolonged PT therefore localises the defect to factor VII, the only factor unique to the extrinsic pathway. Factors VIII and XI prolong only the aPTT, and factor XIII deficiency leaves both screening tests normal because it acts after fibrin monomer formation.

Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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