A 55-year-old man with type II diabetes mellitus, essential hypertension, and ischemic heart disease comes to the physician for a routine examination. Laboratory studies show high serum potassium of 6 mEq/L. He has no muscle weakness and there are no changes in the ECG except for evidence of a prior inferior wall myocardial infarction. There is a mild compensated metabolic acidosis with normal serum anion gap Urine examination shows a significantly lower than expected urine potassium excretion (as measured by daily urine potassium excretion) despite an elevated serum potassium concentration. Which of the following is most likely to explain the hyperkalemia observed in this patient?