A 28-year-old female presents with a 3-week history of itching and burning in the groin fold. On examination, she has a well-demarcated erythematous patch with a raised, scaly border and central clearing in the inguinal region bilaterally. The lesion spares the scrotum. KOH mount shows branching septate hyphae. She has been using a topical corticosteroid cream for 10 days without improvement. What is the most likely diagnosis?

Explanation

## Diagnosis: Tinea Cruris ### Clinical Presentation **Key Point:** Tinea cruris is a dermatophyte infection of the groin, characterized by a well-demarcated erythematous patch with a raised, scaly border and central clearing — the classic "active border" sign. ### Diagnostic Features | Feature | Tinea Cruris | Candidiasis | Erythrasma | |---------|--------------|-------------|------------| | **Border** | Raised, scaly, well-demarcated | Ill-defined, maceration | Sharp, non-scaly | | **KOH mount** | Branching septate hyphae | Budding yeast + pseudohyphae | Negative (bacterial) | | **Spares scrotum** | Yes (typical) | No | No | | **Response to steroids** | Worsens (tinea incognito) | May improve temporarily | Improves | | **Wood's lamp** | Negative | Negative | Coral-red fluorescence | ### Key Point: Tinea Incognito **High-Yield:** Topical corticosteroids suppress inflammation but allow fungal proliferation, creating "tinea incognito" — a modified, less inflammatory presentation. This patient's lack of improvement after steroid use is a red flag for fungal infection. ### Pathophysiology 1. Dermatophyte (usually *Trichophyton rubrum* or *T. mentagrophytes*) invades stratum corneum 2. Inflammatory response creates raised border with active hyphae at margin 3. Central clearing occurs as fungus exhausts local nutrients 4. Groin is predisposed due to warm, moist, occluded environment ### Clinical Pearl **Clinical Pearl:** Tinea cruris typically spares the scrotum (unlike candidiasis, which commonly involves it). This distinction is a key diagnostic clue in groin infections. ### Management - Topical azoles (miconazole, clotrimazole) or terbinafine for 2–4 weeks - Oral terbinafine (250 mg daily × 2–4 weeks) if extensive or refractory - Discontinue corticosteroids immediately - Keep area dry; use antifungal powder [cite:Park 26e Ch 8]