A 25-year-old woman with a history of systemic lupus erythematosus presents with diffuse petechiae and fatigue. Physical examination demonstrates lymphadenopathy and splenomegaly. Laboratory findings include normocytic anemia (hemoglobin = 6.1 g/dL) and thrombocytopenia (30,000/mL). A peripheral blood smear shows polychromasia with 10% reticulocytes. This patient most likely has which of the following hematologic diseases?