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?