Review Question - QID 214730

A 4-year-old girl is brought to the physician by her parents because of yellow discoloration of her skin and eyes for the past week. She was treated 2 weeks ago for a urinary tract infection with antibiotics. She has been experiencing dizziness and fatigue for the past 2 months but is otherwise healthy and up to date on her immunizations. She was born at full-term via spontaneous vaginal delivery without complications. Her father has a history of splenectomy. Her temperature is 98.6°F (37°C), blood pressure is 100/60 mmHg, pulse is 98/min, and respirations are 26/min. On physical exam, she has scleral icterus and pale conjunctivae. She is jaundiced on her trunk and extremities. Her abdomen is soft and nontender. Her spleen is palpated 5 cm below the left costal margin. Laboratory studies demonstrate:

Hemoglobin: 9.2 g/dL
Mean corpuscular volume: 83 µm^3
Mean corpuscular hemoglobin concentration (MCHC): 39% Hb/cell
Leukocyte count: 7,500/mm^3
Platelet count: 200,000/mm^3
Red cell distribution width (RDW): 22%
Total bilirubin: 13.9 mg/dL
Direct bilirubin: 1.9 mg/dL
Lactate dehydrogenase: 450 U/L

A peripheral blood smear is shown in Figure A. Which of the following diagnostic tests would be the most appropriate to aid in the diagnosis?