A 32-year-old woman presents with recurrent deep vein thrombosis at an unusual site. Testing shows resistance to activated protein C in the aPTT-based assay. The underlying mutation most commonly alters which amino acid in the target protein?
- A Lysine 196 in prothrombin
- B Arginine 506 in factor V ✓
- C Cysteine residues of antithrombin
- D Glu 30 in protein S
Correct answer: B. Arginine 506 in factor V
Explanation
Factor V Leiden is a G to B point mutation that replaces arginine 506 with glutamine at one of the cleavage sites for activated protein C. Mutant factor Va resists proteolytic inactivation and persists longer, producing hypercoagulability. It accounts for the vast majority of activated protein C resistance cases. Prothrombin G20210B, antithrombin defects and protein S defects cause thrombophilia but do not give APC resistance.
Reference: Robbins and Cotran Pathologic Basis of Disease, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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