What laboratory value must be closely monitored in a client taking captopril and spironolactone?

The correct choice emphasizes the importance of monitoring serum potassium levels in a client taking captopril and spironolactone due to their effects on potassium balance. Captopril, an ACE inhibitor, can lead to increased potassium retention because it decreases aldosterone levels, which normally promote the excretion of potassium. Spironolactone, a potassium-sparing diuretic, also contributes to higher potassium levels by inhibiting the action of aldosterone at the renal level.

Given these mechanisms, the concurrent use of both medications raises the risk of hyperkalemia, which can lead to serious cardiac events. Therefore, regular monitoring of serum potassium levels is vital to ensure they remain within a safe range and to adjust treatment as necessary.

The other laboratory values listed do not have the same direct relationship with the medications in question. Blood glucose levels are more relevant in the context of diabetes management, bilirubin levels are typically monitored for liver function, and white blood cell count is associated with immune function and infection risk rather than electrolyte management. Thus, monitoring serum potassium is particularly critical for maintaining the safety and effectiveness of therapy with captopril and spironolactone.