A 29-year-old woman presents with headache, confusion and reduced urine output. She is pale and has scattered petechiae; investigations show haemoglobin 78 g/L (115–155), platelet count 14 × 10⁹/L (150–400), creatinine 168 micromol/L (45–90) and fragmented red cells on the blood film. The prothrombin time, activated partial thromboplastin time and fibrinogen concentration are normal, and she has had no diarrhoeal illness. Which one of the following is the most likely diagnosis?