A 35-year-old woman develops painful blue toes and digital ischemia after starting heparin for DVT treatment. Platelet count falls from 280,000 to 55,000/µL over 5 days. The mechanism of this complication is:
- A IgG antibodies against heparin-PF4 complex activating platelets via FcγRIIA, causing consumption and paradoxical thrombosis ✓
- B Direct toxicity of heparin on megakaryocytes reducing platelet production
- C Complement activation by heparin causing platelet lysis
- D Heparin-induced consumption of fibrinogen causing DIC
Correct answer: A. IgG antibodies against heparin-PF4 complex activating platelets via FcγRIIA, causing consumption and paradoxical thrombosis
Explanation
Heparin-induced thrombocytopenia type II (HIT) is caused by IgG antibodies that bind the heparin-platelet factor 4 (PF4) complex. These immune complexes bind FcγRIIA on platelet surfaces, causing massive platelet activation, thrombocytopenia from platelet consumption, and paradoxical thrombosis (white clot syndrome). Direct megakaryocyte toxicity is type I HIT (mild, non-immune); complement-mediated lysis is not the mechanism; DIC has different coagulation abnormalities.
Reference: Robbins & Cotran Pathologic Basis of Disease, 10th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
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