PRESENTING FEATURES: A 47-year-old white woman with a history of stage III squamous cell carcinoma of the anus was transferred to Johns Hopkins Hospital for further evaluation of renal failure, hemolytic anemia, and thrombocytopenia. The patient was first diagnosed with squamous cell carcinoma of the anus 1 year before admission. She was treated with external beam radiation of the pelvis and two cycles of mitomycin C-based chemotherapy (a cumulative dose, 34 mg/m2). Her clinical course was complicated by Clostridium difficile colitis and myositis successfully treated with prednisone. Three months before admission, the patient developed dysuria. Her creatinine increased from normal to 1.7 mg/dL, and microscopic hematuria was present. A renal ultrasound and an abdominal computed tomographic scan showed no abnormalities or obstruction. One month before admission, she underwent a cystoscopy, which showed only radiation-induced changes in the bladder. Two weeks before admission, the patient became delirious and was taken to a hospital, where she was found to be anemic, with a hematocrit level of 23.7%, and thrombocytopenic with a platelet count of 110,000/mm3. Her creatinine level was 5.9 mg/dL. Previous values of hematocrit, platelet count, and serum creatinine were normal. On admission at Johns Hopkins Hospital the patient had no complaints. She was afebrile on physical examination and had normal vital signs. Head, neck, chest, cardiovascular, and abdominal examinations were normal. There was skin pallor, but no echymoses or petechiae. She was alert and oriented with normal mental status. Her neurologic examination was normal. Laboratory data showed a white blood cell count of 6390/mm3, a hematocrit level of 26.5%, and a platelet count of 26,000/mm3. Her blood urea nitrogen level was 57 mg/dL, creatinine level was 4.0 mg/dL, and lactate dehydrogenase was 550 U/L (reference, 115 to 275 U/L). Urinalysis showed innumerable red blood cells and large protein. A peripheral blood smear showed fragmented red blood cells, schistocytes, no abnormal white blood cells, and few platelets. There was no radiographic or clinical evidence of relapse of her squamous cell carcinoma. What is the diagnosis?
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