A 30-year-old woman comes to the emergency department because of weakness and fatigue for 2 days. She has also noticed that her urine is darker than usual. For the past week, she has had a persistent non-productive cough and low-grade fever. She has seasonal allergies. She drinks one to two glasses of wine on social occasions and does not smoke. Her temperature is 37.9°C (100.2°F), pulse is 88/min, respirations are 18/min, and blood pressure is 110/76 mm Hg. She has conjunctival pallor and scleral icterus. Cardiopulmonary examination shows bibasilar crackles. The remainder of the physical examination shows no abnormalities. Laboratory studies show:Leukocyte count 8,000/mm3Hemoglobin 7.1 g/dLHematocrit 21%Platelet count 110,000/mm3MCV 94 μm3Serum Total bilirubin 4.3 mg/dLDirect 1.1 mg/dLIndirect 3.2 mg/dLAST 15 U/LALT 17 U/LLDH 1,251 U/LHaptoglobin 5.8 mg/dL (N = 41-165)An x-ray of the chest shows bilateral patchy infiltrates. A peripheral blood smear shows spherocytes. Which of the following is most likely to confirm the diagnosis?

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