Review Question - QID 108674

A 3-year-old girl is brought to her pediatrician for a 1 month history of fatigue. Her parents report that she has been eating less than usual and is less interested in playing during that time. The patient has no past medical history and her parents say that her only known trigger is an upper respiratory infection a few weeks ago. During this time period, her mother has also noted that her urine appeared abnormally dark. Her parents report recent travel to Ireland to visit family and say that the child was born there. They deny any other travel but endorse contact with a cousin who was diagnosed with infectious mononucleosis 2 months ago. Her temperature is 98.0°F (36.7°C), blood pressure is 98/72 mmHg, pulse is 99/min, and respirations are 22/min. On physical exam, she is noted to have conjunctival pallor and icteric sclera. Laboratory tests are performed and reveal the following:

Leukocyte count: 10,200/mm^3
Hemoglobin: 8.7 g/dL
Hematocrit: 26%
MCV: 94 um^3
Platelet count: 302,000/mm^3
Reticulocyte index (RI): 4.7%

A peripheral smear is performed and the results are shown in Figure A. Which of the following methods is most appropriate for diagnosing this patient’s condition?