A nurse is reviewing the laboratory report for a client who has been taking sodium polystyrene sulfonate. Which of the following findings indicates a therapeutic response to the medication?

Choice A rationale Magnesium levels within the normal range of 1.3 to 2.1 mEq/L do not directly indicate a therapeutic response to sodium polystyrene sulfonate. This medication is primarily used to exchange potassium ions for sodium ions in the gastrointestinal tract, thus lowering serum potassium. Magnesium homeostasis is regulated independently, often by renal excretion and dietary intake, and is not a direct target of this drug. Choice B rationale Calcium levels within the normal range of 9 to 10.5 mg/dL do not indicate a therapeutic response to sodium polystyrene sulfonate. This medication does not significantly impact calcium metabolism. Calcium regulation involves parathyroid hormone and vitamin D, and is separate from the ion-exchange mechanism of sodium polystyrene sulfonate. Choice C rationale Sodium levels within the normal range of 136 to 145 mEq/L, while desirable, do not directly indicate a therapeutic response to sodium polystyrene sulfonate. Although the medication exchanges potassium for sodium, leading to a potential increase in serum sodium, the primary therapeutic goal is potassium reduction, and a normal sodium level alone does not confirm this. Choice D rationale A potassium level of 4.8 mEq/L, which falls within the normal range of 3.5 to 5 mEq/L, indicates a therapeutic response to sodium polystyrene sulfonate. This medication functions as a cation-exchange resin, binding to potassium ions in the intestine and facilitating their excretion, thereby lowering elevated serum potassium levels to a safe range.