A 47-year-old woman is brought to the hospital by her husband with fever, headache, confusion, and jaundice for 1 week. She underwent a hysterectomy 2 months ago and began estrogen replacement therapy recently. On admission, her temperature is 38.7 C (102 F), blood pressure is 140/90 mm Hg, Pulse is 98/min, and Respiratory rate is 20/min. She appears disoriented to time and place. Physical examination reveals jaundiced sclerae and skin, purpura on the trunk, and bleeding gums. Her platelet count is 25,000/mm3, hematocrit is 24%, and creatinine is 4.9 mg/dL. Lactate dehydrogenase (LDH) and indirect bilirubin are elevated. Coagulation tests are within normal limits, but the bleeding time is increased; fibrin-split products and Coombs test are negative. A peripheral blood smear shows schistocytes, helmet-shaped cells, and cells with a triangular shape. Which of the following is the most likely diagnosis?