A nurse is caring for a client who has a new prescription for spironolactone and reports that he forgot to tell the provider that he takes over-the-counter supplements. The nurse should instruct the client to avoid which of the following supplements?

Spironolactone is a potassium-sparing diuretic that works by blocking aldosterone receptors in the distal nephron, causing the kidneys to retain potassium while excreting sodium and water. Because the drug already elevates serum potassium levels, adding potassium supplements creates a significant risk of hyperkalemia, which can cause life-threatening cardiac dysrhythmias, muscle weakness, and cardiac arrest. The nurse must instruct the client to avoid potassium-containing supplements and to limit potassium-rich foods such as bananas, oranges, and potatoes. Calcium supplements do not interact dangerously with spironolactone in this way; calcium does not compound the potassium-retaining mechanism. Magnesium supplements are not contraindicated with spironolactone, as the drug's primary electrolyte concern is potassium retention rather than magnesium accumulation. Iron supplements likewise have no clinically significant interaction with spironolactone that would require avoidance. The key concept is understanding the pharmacodynamics of potassium-sparing diuretics: anything that further raises serum potassium — including OTC potassium supplements, salt substitutes containing potassium chloride, and concurrent use of ACE inhibitors or ARBs — must be avoided to prevent hyperkalemia. This makes potassium the correct and only answer here.