A 28-year-old male farmer from rural Maharashtra presents with a 3-week history of itchy, scaly patches on his groin and inner thighs. Examination reveals well-demarcated erythematous plaques with a raised, active border and central clearing. KOH mount of scale shows branching septate hyphae. The lesions spare the scrotum. What is the most likely causative organism?

Explanation

## Clinical Diagnosis: Tinea Cruris ### Key Clinical Features **Key Point:** Tinea cruris (jock itch) is the most common dermatophytosis in warm, humid climates like India. The characteristic presentation includes: - Well-demarcated erythematous plaques with active raised border - Central clearing ("ringworm" appearance) - Pruritic, scaly lesions in intertriginous areas (groin, inner thighs) - **Sparing of scrotum and genitalia** — this is pathognomonic and helps differentiate from candidiasis ### Causative Organisms in Tinea Cruris | Organism | Prevalence in India | Clinical Notes | |----------|-------------------|----------------| | **Trichophyton rubrum** | Most common (60–70%) | Anthropophilic; warm climate preference; causes chronic infections | | Trichophyton mentagrophytes | 15–20% | Zoophilic; associated with animal contact | | Epidermophyton floccosum | 10–15% | Anthropophilic; less common in tinea cruris | | Microsporum canis | Rare | Zoophilic; more common in tinea corporis; animal exposure history | **High-Yield:** *T. rubrum* is the **most common dermatophyte in India** overall and specifically the leading cause of tinea cruris in warm, humid regions. It is anthropophilic (human-to-human transmission) and thrives in moist intertriginous areas. ### Diagnostic Confirmation **Clinical Pearl:** KOH mount showing **branching septate hyphae** confirms dermatophyte infection. The **sparing of scrotum** is crucial — candidiasis (Candida albicans) typically involves the scrotum and genitalia, whereas dermatophytes spare these areas. ### Why This Patient Has T. rubrum Infection 1. **Epidemiology:** Farmer in rural Maharashtra (warm, humid climate) — ideal for T. rubrum 2. **Site:** Groin and inner thighs — classic tinea cruris location 3. **Morphology:** Active border with central clearing — typical of T. rubrum 4. **Sparing of scrotum:** Rules out candidiasis; consistent with dermatophyte 5. **KOH findings:** Branching septate hyphae — diagnostic of dermatophyte **Mnemonic:** **TRAM** — *T. rubrum* is the most common cause of **Tinea** in **Warm climates** and **Anthropophilic** transmission.