A 58-year-old man with hypertension and chronic kidney disease (eGFR 35 mL/min/1.73 m²) presents with ankle edema and dyspnea on exertion. He is currently on lisinopril and amlodipine. His serum potassium is 5.2 mEq/L (normal 3.5–5.0) and serum creatinine is 2.1 mg/dL. The cardiologist prescribes a loop diuretic for fluid overload. Which diuretic would be most appropriate, and why should a potassium-sparing agent be avoided in this patient?