A 50-year-old diabetic patient presents with thickened, discolored, and brittle toenails, consistent with onychomycosis. A fungal culture grows a dermatophyte that typically produces a deep red pigment on potato dextrose agar and tear-drop shaped microconidia along the hyphae. Which of the following is the most likely causative agent?

Explanation

Trichophyton rubrum is the most common cause of onychomycosis (tinea unguium) and tinea pedis. Its characteristic cultural features include the production of a deep red pigment on the reverse side of potato dextrose agar (or other media) and the presence of tear-drop or peg-shaped microconidia arranged along the sides of the hyphae. Microsporum canis is a common cause of tinea capitis and produces macroconidia that are spindle-shaped with rough walls. Epidermophyton floccosum causes tinea cruris and tinea pedis, and produces club-shaped macroconidia in clusters, but no microconidia. Trichophyton mentagrophytes causes tinea pedis and tinea corporis, and produces spiral hyphae and grape-like clusters of microconidia.