A 25-year-old woman presents with intermittent episodes of muscle weakness, cramps, and fatigue over the past few months. She denies any significant past medical history and does not take any regular medications. Her physical exam is unremarkable except for mild lower extremity weakness. Laboratory evaluation reveals: • Serum potassium: 2.9 mEq/L (normal: 3.5–5.0 mEq/L) • Serum magnesium: 1.2 mg/dL (normal: 1.7–2.2 mg/dL) • Serum bicarbonate: 32 mEq/L (normal: 22–28 mEq/L) • Urinary calcium excretion: Low Which of the following is the most likely diagnosis in this patient?