A 68-year-old woman is brought to the emergency department. Her daughter witnessed her faint twice at home today. Each episode involved a sudden loss of consciousness lasting about one minute, with spontaneous recovery. She has a past medical history of major depressive disorder and hypertension. Her medications include citalopram 40 mg daily and hydrochlorothiazide 25 mg daily. For the last three days, she has been experiencing watery diarrhea and poor oral intake.

While being evaluated in the emergency department, she has another syncopal episode. The cardiac monitor shows an episode of polymorphic ventricular tachycardia, which terminates spontaneously after about 30 seconds. Her vital signs are stable between these episodes.

Initial laboratory studies are as follows:

WBC: 9,800/mm³ (4,500-10,000)

Hemoglobin: 14.1 g/dL (12.0-16.0)

Platelets: 250,000/mm³ (150,000-400,000)

BUN: 30 mg/dL (7-18)

Creatinine: 1.3 mg/dL (0.6-1.1)