A 42-year-old woman is evaluated in the emergency department for an episode of transient loss of consciousness. Two hours ago, she was standing in line at the grocery store and began to feel warm, flushed, and nauseated. After about a minute, she remembers becoming lightheaded with “graying” of her vision, after which she fell to the ground. She immediately regained consciousness and had no residual symptoms. She reports no chest pain, palpitations, nausea, numbness, tingling, weakness, vertigo, or headache. There was no apparent seizure activity or bowel or bladder incontinence.
On physical examination, temperature is 36.8 °C (98.2 °F). Blood pressure is 110/80 mm Hg supine and 102/78 mm Hg standing, pulse rate is 78/min supine and 88/min standing, and respiration rate is 14/min. BMI is 28. The lungs are clear, and examination of the heart is normal and without murmurs. The remainder of the general medical examination is unremarkable. Neurologic examination is normal.
Laboratory test results reveal a hemoglobin level of 13.2 g/dL (132 g/L). Pregnancy test is negative.
Which of the following is the most appropriate diagnostic test to perform next?