Which of the following is NOT an anti-fungal agent?

Acyclovir is not an anti-fungal agent; it is classified as an antiviral medication. Specifically, acyclovir is used primarily to treat infections caused by certain types of viruses, such as herpes simplex virus and varicella-zoster virus. Its mechanism of action involves inhibiting viral DNA synthesis, which makes it effective against these viral infections but ineffective against fungal infections.

In contrast, nystatin, fluconazole, and ketoconazole are all antifungal agents that are used to treat various fungal infections. Nystatin is often used for oral candidiasis, fluconazole is a systemic antifungal that treats infections like candidiasis and cryptococcal meningitis, and ketoconazole can be used for skin infections like tinea and for systemic fungal infections as well. Each of these agents works through mechanisms aimed at disrupting cell membrane integrity or inhibiting fungal cell metabolism, underlining their effectiveness in targeting fungal pathogens.