<s>[INST] context: Warfarin is a coumarin anticoagulant used for the prophylaxis and treatment of thromboembolic complications associated with cardiac valve replacement and atrial fibrillation, as well as the prophylaxis and treatment of venous thrombosis and pulmonary embolism. Warfarin may cause necrosis of the skin (typically on the breasts, thighs, and buttocks) generally between the third and tenth days of therapy. The lesions are initially sharply demarcated, erythematous, and purpuric. They may resolve or progress to large, irregular, hemorrhagic bullae that can eventually lead to necrosis. The mechanism for this reaction is related to warfarin's ability to deplete protein C, which can lead to a state of hypercoagulability and thrombosis in the cutaneous microvasculature. Aspirin is commonly used for its antiplatelet effect; however, it would not be indicated for anticoagulation of a patient with a recent cardiac valve replacement. Fuhermore, aspirin is not associated with the development of this type of skin necrosis. Cefazolin is a first-generation cephalosporin antibiotic commonly used as a perioperative prophylactic agent. If the patient was allergic to this antibiotic, an erythematous rash might have appeared. However, the rash would not lead to the appearance of large, hemorrhagic bullae. Heparin is an intravenous anticoagulant indicated for the prophylaxis and treatment of thromboembolic complications associated with cardiac valve replacement and atrial fibrillation. It is also indicated for the prophylaxis and treatment of venous thrombosis, pulmonary embolism, and for treatment of some coagulopathies. Although heparin is associated with the development of thrombocytopenia, it is not associated with skin necrosis. [/INST] question: A 58-year-old woman had a mitral valve replacement, and was placed on anticoagulants and prophylactic antibiotics following her surgery. Five days after her surgery, she developed a sharply demarcated, erythematous rash on her left thigh. Two days after the rash appeared, large hemorrhagic bullae began to form in the area of the rash. Which of the following medications most likely caused the patient's rash? | options: A: Aspirin B: Cefazolin C: Heparin D: Warfarin | answer: D </s>