A 20-year-old woman with fatigue and palpitations.

A 20-year-old woman presents to the emergency department with fatigue and the sudden onset of palpitations. She reports no history of significant illness or surgery. She says she is not currently taking prescription or over-the-counter medications. She does not smoke, drink alcohol, or use illicit drugs. Her weight is 52 kg (115 lb), her height is 170 cm (67 in), and her body mass index (BMI) is 18 kg/m2. Vital signs: temperature 35.7°C (96.4°F), blood pressure 92/48 mm Hg, heart rate 73 bpm, respiratory rate 5 breaths per minute, and oxygen saturation 98% on room air. She appears tired but is alert, conversant, and cooperative. Her skin is normal, and dentition is fair. Her pulse is regular, and respirations are slow. The abdomen is soft, nontender, and flat. Strength is 4 on a scale of 5 in all extremities. Deep-tendon reflexes are 2+ and symmetric. Electrocardiography (FIGURE 1) in the emergency department shows ST-segment depression, a prolonged corrected QT interval of 665 msec, T-wave inversion, PR prolongation, increased P-wave amplitude, and U waves.