A 28-year-old woman presents with fatigue for 6 months and 3 episodes of tetany with flaccid paralysis. Examination: alert, coherent speech. Limbs show involuntary movements, muscle strength grade II, no significant muscle atrophy. BP 180/110 mmHg, heart rate 92/min, regular rhythm. Lab results: serum potassium 2.5 mmol/L, 24-hour urinary potassium 30 mmol/L. Which test should be performed next for definitive diagnosis?