Review Question - QID 212896

A 27-year-old woman presents to the emergency department complaining of a left-sided headache and right-sided blurry vision. She states that 2 weeks ago she developed dark urine and abdominal pain. She thought it was a urinary tract infection so she took trimethoprim-sulfamethoxazole that she had left over. She planned on going to her primary care physician today but then she developed headache and blurry vision so she came to the emergency department. The patient states she is otherwise healthy. Her family history is significant for a brother with sickle cell trait. On physical examination, there is mild abdominal tenderness, and the liver edge is felt 4 cm below the right costal margin. Labs are drawn as below:

Hemoglobin: 7.0 g/dL
Platelets: 149,000/mm^3
Reticulocyte count: 5.4%
Lactate dehydrogenase: 3128 U/L

Total bilirubin: 2.1 mg/dL
Indirect bilirubin: 1.4 mg/dL
Aspartate aminotransferase: 78 U/L
Alanine aminotransferase: 64 U/L

A peripheral smear shows polychromasia. A Doppler ultrasound of the liver shows decreased flow in the right hepatic vein. Magnetic resonance imaging of the brain is pending. Which of the following tests, if performed, would most likely identify the patient’s diagnosis?