A 7-year-old boy with a history of hereditary spherocytosis presents to the emergency department with sudden onset of pallor, fatigue, and lethargy. His parents report that he had a mild fever and runny nose a week ago, which resolved without treatment. On examination, he appears pale, and laboratory tests reveal a significant drop in hemoglobin levels with a markedly low reticulocyte count. Which of the following is the most likely cause of this patient’s current hematologic findings?
