A 72-year-old woman with heart failure (LVEF 35%) and hypertension is started on lisinopril and furosemide 40 mg daily. After 2 weeks, she develops severe muscle cramps, fatigue, and generalized weakness. Laboratory findings: Na⁺ 128 mEq/L, K⁺ 2.9 mEq/L, Cl⁻ 92 mEq/L, HCO₃⁻ 32 mEq/L. ECG shows U waves and flattened T waves. Which electrolyte abnormality is the primary driver of her symptoms, and what is the most appropriate next step?