A 3-week-old female with history of premature birth was found to have worsening anemia. After birth, she had a normal hemoglobin of 16.0 g/dL. Over the subsequent three weeks, her hemoglobin trended down to 6.8 g/dL. White blood cell and platelet counts were unremarkable. Reticulocyte percentage was 10.7%. Her indirect bilirubin was elevated at 13.2 mg/dL. The peripheral smear demonstrated the finding as below.