A nurse is reviewing the medication administration record of a client who is to undergo excretory urography using contrast dye. The nurse should plan to withhold which of the following medications 24 to 48 hr before the procedure?

Metformin must be withheld 24 to 48 hours before any procedure involving iodinated contrast dye, such as excretory urography. The rationale is that contrast agents can cause acute kidney injury, and if renal function deteriorates, metformin can accumulate to toxic levels and precipitate lactic acidosis — a rare but life-threatening complication. By holding metformin before the procedure and typically for 48 hours afterward (until renal function is confirmed to be normal), this risk is eliminated. Atenolol is a beta-blocker used for hypertension and angina; there is no interaction with contrast dye that necessitates withholding it, and abrupt discontinuation could cause rebound hypertension or cardiac events. Acetaminophen has no clinically significant interaction with iodinated contrast and does not require withholding. Prednisone is a corticosteroid that is sometimes continued or even given prophylactically before contrast procedures in clients with allergy histories, but it does not require routine withholding. Metformin is the only medication among these options with a well-documented, guideline-supported requirement to hold before contrast-enhanced imaging studies.