Which of the following may increase serum lithium levels when taken with ACE inhibitors?

The correct answer is potassium supplements, as the interaction between potassium supplements and ACE inhibitors can lead to increased serum lithium levels. ACE inhibitors can decrease renal clearance of lithium, making it more concentrated in the blood. When potassium supplements are added to this combination, they can further disrupt the kidney's ability to manage both potassium and lithium levels, resulting in elevated serum lithium levels.

This is particularly important for patients on lithium therapy, as high lithium levels can lead to toxicity, causing symptoms such as tremors, confusion, or even more severe neurological effects. Monitoring serum lithium levels closely is essential when potassium supplementation is introduced in patients using ACE inhibitors.

Other options, while they have their respective roles in pharmacology and may influence blood pressure or electrolyte balance, do not directly lead to the same increase in serum lithium levels in the context of ACE inhibitors as potassium supplements do.