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HIT risk arises when the patient is exposed to heparin, because antibodies develop against the PF4–heparin complex that activate platelets and can cause both thrombocytopenia and thrombosis. This is most classically seen with unfractionated heparin used to treat conditions like deep vein thrombosis. The other scenarios do not involve this specific immune reaction to heparin: warfarin therapy can cause issues like skin necrosis but not HIT; placental abruption is an obstetric emergency unrelated to HIT; systemic lupus erythematosus can involve immune-mediated processes but not the PF4–heparin antibody–driven HIT mechanism. Therefore, heparin therapy for a DVT best indicates a risk for HIT.