A 72-year-old woman with hypertension and stage 4 CKD (eGFR 22 mL/min/1.73m²) is admitted with a 3-day history of nausea, fatigue, and constipation. She has been on amlodipine, enalapril, and atorvastatin for 5 years. Her grandson recently gave her NSAIDs for back pain, which she took for 2 days. Laboratory findings: K⁺ 6.8 mEq/L, Na⁺ 138 mEq/L, Cl⁻ 104 mEq/L, HCO₃⁻ 18 mEq/L, creatinine 3.2 mg/dL, pH 7.28, PaCO₂ 32 mmHg. ECG shows peaked T waves but no PR prolongation or QRS widening. Which of the following is the MOST important step in preventing recurrence of hyperkalemia in this patient?