A father brings his 3-year-old daughter to the emergency center after noting her to be pale and tired and with a subjective fever for several days. Her past history is significant for an upper respiratory infection 4 weeks prior, but she had been otherwise healthy. The father denies emesis or diarrhea but does report his daughter has had leg pain over the previous week, waking her from sleep. He also reports that she has been bleeding from her gums after brushing her teeth. Examination reveals a listless pale child. She has diffuse lymphadenopathy with splenomegaly but no hepatomegaly. She has a few petechiae scattered across her face and abdomen and is mildly tender over her shins but does not have associated erythema or joint swelling. A CBC reveals a leukocyte count of 8,000/μL with a hemoglobin of 4 g/dL and a platelet count of 7,000/μL. The automated differential reports an elevated number of atypical lymphocytes. Which of the following diagnostic studies is the most appropriate next step in the management of this child?
a) Bone marrow aspiration
b) Coombs test
c) Peripheral blood smear
d) Serum iron studies