A 69-year-old Woman with chronic liver disease treated with furosemide and potassium chloride for ascites and peripheral edema presents with weakness and fatigue. Her serum laboratory Values are:
* Sodium: 138 mEq/L
* Potassium: 2.6 mEq/L
* Glucose: 88 mg/dL
* BUN: 33 mg/dL
* Chloride: 101 mEq/L
* HCO3- 26 mEq/L
* Creatinine: 1.1 mg/dL
Attempts to increase her serum potassium over the next 48 hours include intravenous replacement (240 mEq over this period), without success. The current serum potassium concentration is 2.8 mEq/L. Which of the following is the most appropriate test to obtain for this patient?