The correct answer is F. The key data to make a correct diagnosis of thrombotic thrombocytopenic purpura (TTP) include the following: severe thrombocytopenia, which results in a bleeding diathesis; elevated indirect bilirubin and high LDH with schistocytes in the blood smear, indicating microangiopathic hemolytic anemia; renal dysfunction (high creatinine); and neurologic and systemic symptoms (headache, confusion, and fever).

Negative findings important to rule out similar conditions include a negative Coombs test and absence of fibrin split products. Thrombotic thrombocytopenic purpura (TTP) is a disorder of unclear pathogenesis, perhaps related to circulating platelet-agglutinating factors. It presents with a characteristic combination of microangiopathic hemolytic anemia, fever without infection, neurologic symptoms, bleeding diathesis secondary to thrombocytopenia, and renal impairment. This condition may be precipitated by pregnancy or use of estrogens.

Disseminated intravascular coagulation (DIC; choice A) can be differentiated from TTP because of abnormal coagulation tests. In DIC, microangiopathic hemolysis is also present, but prothrombin time (PT) is prolonged, fibrinogen levels are reduced, and fibrin split products are elevated.

Evans syndrome (choice B) refers to coexistence of autoimmune hemolytic anemia (positive Coombs test), and autoimmune thrombocytopenic purpura (see choice D).

Hemolytic-uremic syndrome (HUS; choice C) is not significantly different from TTP. The two conditions, in fact, are considered manifestations of the same pathogenetic spectrum. However, the vascular bed of the CNS is not involved in HUS; thus, mental status changes are not part of the clinical picture.

Idiopathic (autoimmune) thrombocytopenic purpura (ITP; choice D) is an immune disorder caused by autoantibodies to platelet antigens. Systemic illness is not present in ITP, which is characterized by isolated thrombocytopenia without other hematologic abnormalities. Ten percent of cases will manifest in association with autoimmune hemolytic anemia (Evans syndrome).

Malignant hypertension (choice E) may cause microangiopathic hemolytic anemia. However, blood pressure values would be extremely elevated.