A 2-year-old child presents with severe pallor and fatigue. Lab work reveals a normocytic anemia with a hemoglobin of 5.5 g/dL and marked reticulocytopenia. A bone marrow aspirate shows a selective reduction in red blood cell precursors. Which of the following findings would most strongly support a diagnosis of Diamond-Blackfan anemia (DBA) over transient erythroblastopenia of childhood (TEC)?