Tinea incognito caused by Trichophyton mentagrophytes complex: a case report

Background Tinea incognito is a dermatophyte infection that loses its classic, pathognomonic diagnostic morphology due to the use of topical corticosteroids or systemic immunosuppression. This morphologic distortion frequently leads to diagnostic delays, inappropriate treatment escalation, and prolonged patient distress. Case Summary A woman in her late 60s presented with a 10-month history of a worsening, intensely pruritic, and painful circular erythematous plaque on her right leg following a minor environmental skin abrasion. The lesion was initially diagnosed to be a bacterial infection with administration of antibiotic therapy. She was subsequently treated with three increasingly potent topical corticosteroids (mometasone furoate, fluocinonide, and clobetasol propionate), which caused paradoxical worsening. While standard fungal cultures from an incisional biopsy yielded no growth, histopathology via Gomori methenamine silver staining revealed deep fungal elements. A panfungal polymerase chain reaction (PCR) assay amplifying the internal transcribed spacer 2 region, followed by Nanopore DNA sequencing and comparison against a curated reference database, identified Trichophyton mentagrophytes complex; species-level resolution was not achievable by this method. The patient was successfully cured with a 12-week course of systemic antifungal therapy, resulting in complete clinical resolution. Conclusion This case highlights tinea incognito as a significant diagnostic challenge. When inflammatory skin lesions paradoxically expand with topical steroid therapy, clinicians must maintain a high index of suspicion for a fungal etiology. Because conventional fungal cultures are frequently negative in this context, molecular diagnostics are valuable; dermatophyte-targeted assays applied to skin specimens are the preferred first-line molecular approach, with panfungal PCR/sequencing on tissue reserved for histology-positive, culture-negative cases.