This case highlights several important clinical and diagnostic lessons:

1) Misleading Biopsy Results: A shave biopsy from the center of the lesion showed only actinic keratosis and stasis changes. Dermatophytes often localize at the advancing margin, so central biopsies can miss the infection. Fungal organisms may also be subtle or absent on routine H&E staining unless special stains (PAS/GMS) are requested.

2) Treatment Pitfalls: A trial of 5-fluorouracil was ineffective because the underlying process was not actinic neoplasia. Subsequent use of clobetasol caused significant worsening. Corticosteroids suppress local immune responses, alter clinical morphology, and allow fungal proliferation. This steroid-altered presentation is known as tinea incognito.

3) Key Clinical Clues: Lack of improvement with standard treatments (5-FU, corticosteroids) should prompt reconsideration of the diagnosis. Worsening with steroids is a red flag that suggests an infectious rather than inflammatory etiology. Chronic plaques on the leg of elderly patients are often attributed to stasis dermatitis or eczema, but the unilateral distribution and treatment resistance here were important diagnostic hints.

4) Diagnostic Approach: KOH preparation of skin scrapings remains a rapid, inexpensive, and reliable test for dermatophyte infection. Always consider performing KOH when an “eczema-like” lesion is atypical or unresponsive to therapy.

5) Management Principles: Once confirmed, systemic antifungal therapy (e.g., oral terbinafine or itraconazole) is preferred for extensive or refractory cases. Topical antifungals may suffice for localized disease but are usually inadequate for widespread tinea incognito. Corticosteroids should be avoided until the fungal infection is cleared.

Take-home message:

Not all chronic, erythematous plaques on the legs of elderly patients are stasis dermatitis or nummular eczema, and when there is a mismatch between the biopsy and the clinical picture, you should reconsider your diagnosis. Always keep tinea incognito in the differential, especially when lesions worsen with corticosteroids. A simple KOH prep can prevent weeks of mismanagement and direct the patient toward curative therapy.