A 19-year-old man presents to the emergency department after a syncopal episode. He was watching television when he suddenly lost consciousness, slumping over in his chair. A family member witnessed the event, stating he was unresponsive for about 20 seconds. He had no seizure-like activity. He quickly regained consciousness and felt well, though he was confused about the event. He denies any preceding symptoms such as chest pain, shortness of breath, palpitations, or lightheadedness. He has no significant past medical history and takes no medications. He is a college student and plays recreational basketball. His family history is notable for an uncle who died suddenly in his sleep at age 25. The cause was undetermined. He denies smoking, alcohol, and illicit drug use.

Initial laboratory studies are as follows:

WBC: 8,500/mm³ (4,500-10,000)

Hemoglobin: 15.1 g/dL (13.5-17.5)

Hematocrit: 45% (41-53)

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

Na⁺: 140 mEq/L (136-146)

K⁺: 4.1 mEq/L (3.5-5.0)

Cl⁻: 101 mEq/L (95-105)

HCO₃⁻: 25 mEq/L (22-28)

BUN: 15 mg/dL (7-18)

Creatinine: 1.0 mg/dL (0.6-1.2)

Calcium: 9.5 mg/dL (8.4-10.2)

Magnesium: 2.0 mg/dL (1.8-2.6)

Glucose: 90 mg/dL (70-110)

Troponin T: <0.01 ng/mL (<0.01)