A 28-year-old, previously healthy man has noted increasing fatigue for the past 6 months and the formation of bruises after minimal trauma. Over the past 2 days, he has developed a cough. On physical examination, his temperature is 38.9deg C, and he has diffuse rales in both lungs. He has no hepatosplenomegaly and no lymphadenopathy. Laboratory findings include a sputum culture positive for Streptococcus pneumoniae, hemoglobin of 7.2 g/dL, hematocrit of 21.7%, platelet count of 23,400/ mm3, WBC count of 1310/ mm3, prothrombin time of 13 seconds, partial thromboplastin time of 28 seconds, and total bilirubin of 1 mg/dL. The ANA test result is negative. What is the most likely explanation of these findings?

Aplastic anemia leads to marked pancytopenia. Many cases are idiopathic, although some can follow toxic exposures to chemotherapy drugs or chemicals, such as benzene. Some cases may follow viral hepatitis infections. An intrinsic defect in stem cells, or T lymphocyte suppression of stem cells, can play a role in the development of aplastic anemia. Hemolysis is unlikely because the bilirubin is normal, and there is no history of an autoimmune disease. Increased susceptibility to complement lysis occurs in paroxysmal nocturnal hemoglobinuria as a result of mutations in the PIGA gene. It is unlikely that the patient has the metastatic disease at this age, with no prior illness; metastases are more likely to produce a leukoerythroblastic peripheral blood appearance. Sequestration of peripheral blood cells in an enlarged spleen could account for mild pancytopenia, but in this case, the spleen is not enlarged.