A patient taking spironolactone has a follow-up appointment. Which of the following laboratory values would be most important to monitor?

Rationale: A. Spironolactone is a potassium-sparing diuretic and does not typically affect calcium levels. Monitoring calcium is not a priority unless the client has another condition affecting calcium metabolism. B. Spironolactone does not directly affect WBC counts. Monitoring WBCs is unrelated to this medication unless the client is taking additional drugs that impact bone marrow or immune function. C. Spironolactone does not influence thyroid function, so assessing TSH or thyroid hormone levels is not necessary for routine follow-up. D. Spironolactone is a potassium-sparing diuretic, which can increase serum potassium, potentially leading to hyperkalemia, a life-threatening condition. Monitoring potassium levels is essential to ensure safe use, particularly in clients with kidney impairment or those taking other potassium-increasing medications.