Which drug is incorrectly associated with treating aspergillosis?

Flagyl, or metronidazole, is primarily an antibiotic used to treat a variety of infections, particularly those caused by anaerobic bacteria and certain parasites. It is not effective against fungal infections such as aspergillosis, which is caused by a type of mold belonging to the Aspergillus species. The treatment for aspergillosis typically involves antifungal medications rather than antibiotics.

In contrast, the other drugs listed may have roles in treating aspergillosis. Dexamethasone is a corticosteroid that can be used to reduce inflammation in conjunction with antifungal treatment. Amoxicillin is an antibiotic, but it's not directly used for fungal infections. Tylosin, an antibiotic commonly used in veterinary medicine, can serve a role in treating certain conditions associated with infections in birds but does not target the fungus responsible for aspergillosis either.

Thus, flagyl stands out as the option that is incorrectly associated with the treatment of this fungal condition.