Which medication taken by a patient with decreased renal function will be of most concern to the nurse?

A. Folic acid (vitamin B5): Folic acid is a water-soluble vitamin that supports red blood cell production and is often supplemented in patients with renal disease due to losses during dialysis. It does not harm kidney function and is generally considered safe. B. Penicillin (Bicillin C-R): Penicillin is excreted by the kidneys, so dosage adjustments may be required in renal impairment. However, it is not as directly nephrotoxic as some other medications and can usually be managed safely with monitoring. C. Warfarin (Coumadin): Warfarin metabolism occurs primarily in the liver, not the kidneys. While close monitoring of INR is always required, renal impairment does not directly contraindicate its use or pose high nephrotoxic risk. D. Ibuprofen (Motrin): NSAIDs such as ibuprofen decrease renal blood flow by inhibiting prostaglandin synthesis, which can significantly worsen kidney function. In patients with decreased renal function, they increase the risk of acute kidney injury and fluid retention.