A 35-year-old immunocompromised patient on long-term corticosteroids develops extensive tinea corporis that fails to respond to topical terbinafine. Nail clippings sent for culture grow a dermatophyte with a non-pigmented, powdery colony. Which species should be suspected and why might terbinafine fail?
- A Trichophyton rubrum — squalene epoxidase mutation conferring terbinafine resistance ✓
- B Microsporum gypseum — natural resistance to allylamines
- C Epidermophyton floccosum — inherent resistance to all azoles
- D Trichophyton tonsurans — only sensitive to griseofulvin
Correct answer: A. Trichophyton rubrum — squalene epoxidase mutation conferring terbinafine resistance
Explanation
Trichophyton rubrum is the most common cause of chronic/extensive tinea and onychomycosis. Terbinafine resistance in T. rubrum is increasingly recognized, caused by point mutations in the squalene epoxidase gene (SQLE) — the target enzyme of allylamines. In such cases, itraconazole or voriconazole may be used. Microsporum gypseum is a geophilic dermatophyte; Epidermophyton floccosum does not infect hair but is generally susceptible to allylamines. Terbinafine resistance in T. rubrum is a current emerging clinical concern in dermatology.
Reference: Neena Khanna Illustrated Synopsis of Dermatology & STD, 6th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.