A client diagnosed with trichomoniasis is prescribed metronidazole (Flagyl). The nurse should instruct the client to avoid which contraindication associated with this medication?

A. Grapefruit juice does not have a known interaction with metronidazole and does not need to be avoided. B. Dairy does not interfere with metronidazole's absorption or effectiveness, so it is not a contraindication. C. Alcohol should be strictly avoided during and for at least 48 hours after metronidazole treatment, as it can cause a severe reaction, including nausea, vomiting, flushing, and rapid heart rate. D. Tyramine-based foods do not interact with metronidazole, so they are not a contraindication for this medication.