Review Question - QID 213774

A 3-year-old boy presents to the emergency department with a 1 week history of fever and lethargy. His parents say that they assumed he had a common viral infection but became worried when he seemed to be more irritable and pale compared to normal. His past medical history is significant for trisomy 21 and allergies. Physical exam reveals petechiae and purpura diffusely as well as gingival hyperplasia. The most likely etiology of this patient's symptoms involves proliferation of which of the following cell types?