Q. A 41-year-old female presents to your clinic with complaints of dizziness and lightheadedness. She reports the symptoms occur frequently and believes they are associated with foods that she eats. There is no reported loss of consciousness or trauma. She denies any associated symptoms. The patient consumes coffee each morning and suggests caffeine as a possible etiology. She does not smoke tobacco or consume alcohol regularly. Her past medical history is unremarkable and she does not take any prescribed medications. On physical examination, she is afebrile, her blood pressure is 130/85 mm Hg, heart rate 70/min, respiratory rate 18/min, and oxygen saturation in 99 percent on room air. A neurologic assessment demonstrates no deficits. Her neck is supple with no masses or bruits. Auscultation of her chest reveals clear breath sounds bilaterally, and her heart is in regular rate and rhythm with no murmurs, rubs, or gallops appreciated. Her abdomen is soft, non-tender, non-distended, with no masses or organomegaly. You order an electrocardiogram (see image), and the patient becomes symptomatic following the procedure. You review the results and decide to order blood work as well. Laboratory Results Leukocyte count: 4,500/mm3 Hemoglobin: 13 g/dL Hematocrit: 38 percent Platelet count: 250,000/mm3 Blood Urea Nitrogen: 15 mg/dL Creatinine: 1.0 mg/dL Glucose: 120 mg/dL Thyroid Stimulating Hormone: 1.0 uU/mL Based on this patient’s history, physical examination, and diagnostic work-up, which of the following is the most likely underlying etiology of her symptoms?