A nurse receives shift report on a patient with renal failure who has received sodium polystyrene. The nurse reviews the patient's chart to evaluate the effectiveness of the medication. Which laboratory result would indicate to the nurse that the medication has been effective?

A. A serum phosphorus level of 3.4 mEq/L (reference range 2.5-4.5 mEq/L): Phosphorus levels are unrelated to the mechanism of sodium polystyrene sulfonate, which is used to lower potassium, not phosphate. Normal phosphorus does not indicate effectiveness of this medication. B. A serum potassium level of 4.0 mEq/L (reference range 3.5-5.0 mEq/L): Sodium polystyrene sulfonate binds potassium in the intestines and promotes its excretion in the stool, reducing serum potassium. A potassium level of 4.0 mEq/L indicates that hyperkalemia has resolved and the medication has achieved its therapeutic goal. C. A serum calcium level of 8.5 mg/dL (reference range 8.5-10.2 mg/dL): Calcium levels are not directly affected by sodium polystyrene. While important in renal failure, a normal calcium level does not reflect the effectiveness of potassium-lowering therapy. D. A serum sodium level of 145 mEq/L (reference range 135-145 mEq/L): Sodium levels may rise slightly with sodium polystyrene administration, but the primary therapeutic target is potassium. Normal sodium alone does not confirm the drug’s effectiveness in treating hyperkalemia.