A 60-year-old man with chronic kidney disease takes over-the-counter magnesium citrate 800 mg/day to treat constipation in addition to a magnesium-containing multivitamin. Critically evaluate his risk for magnesium toxicity and recommend an evidence-based course of action.

• A. Increase dietary salt to counteract magnesium accumulation; chloride intake will bind magnesium in the gut and prevent absorption, so adding table salt is the best immediate solution.
• B. High risk for hypermagnesemia because impaired renal function reduces magnesium excretion and daily supplemental magnesium (total >350 mg supplemental) can accumulate; stop or reduce supplements, consult physician for serum magnesium monitoring and consider alternate constipation treatments.
• C. No action is necessary because magnesium toxicity only occurs with intravenous administration during hospitalization and renal disease does not affect oral absorption.
• D. Low risk because magnesium from supplements is not absorbed when kidney function is poor, so continuing high-dose magnesium is safe and beneficial for heart health.

Answer: (B)