Identify the antifungal agent used to treat onychomycosis (nail fungus).

Terbinafine is the antifungal agent primarily used to treat onychomycosis, which is a fungal infection of the nails. It works by inhibiting the synthesis of ergosterol, an essential component of fungal cell membranes, thereby compromising the structural integrity of the fungus and leading to its death. Terbinafine is particularly effective against dermatophytes, the most common cause of onychomycosis, and has a high cure rate with relatively few side effects compared to other antifungals.

Griseofulvin, while used for certain fungal infections, is more effective for dermatophyte infections of the skin and hair rather than nails. Caspofungin is an echinocandin antifungal effective against systemic fungal infections, particularly Candida and Aspergillus species, but not typically used for superficial infections like onychomycosis. Ketoconazole is an azole antifungal that can be used for various fungal infections, but its use in onychomycosis is limited due to concerns regarding safety and efficacy compared to terbinafine.

In summary, the effectiveness of terbinafine against the specific organisms that cause onychomycosis, along with its favorable side effect profile, makes it the first-line