A 58-year-old man with chronic kidney disease (eGFR 35 mL/min/1.73m²) and hypertension is started on a loop diuretic for fluid overload. After 2 weeks of therapy, he develops severe muscle cramps, generalized weakness, and palpitations. Serum potassium is 2.8 mEq/L. Which investigation would be most appropriate to assess the mechanism of hypokalemia and guide further management?