A 30-year-old woman complains of recent easy fatigability, bruising, and recurrent throat infections. Physical examination reveals numerous petechiae over her body and mouth. Abnormal laboratory findings include hemoglobin of 6 g/dL, WBC of 1,500/mL, and platelets of 20,000/mL. The bone marrow is hypocellular and displays increased fat. What is the appropriate diagnosis?