A 28-year-old male farmer from rural Maharashtra presents with a 3-week history of itchy, scaly patches on his right groin and inner thigh. The lesions are well-demarcated, erythematous, with a raised border and central clearing. He denies systemic symptoms. KOH mount of scale shows septate hyphae. Which of the following is the most likely causative organism?

Explanation

## Diagnosis: Tinea Cruris (Jock Itch) ### Clinical Presentation The patient presents with classic features of tinea cruris: - Well-demarcated erythematous patches with raised borders and central clearing ("ringworm" appearance) - Intense pruritus localized to groin and inner thigh (warm, moist intertriginous area) - Risk factors: male sex, warm climate, occupational exposure (farmer) - KOH mount showing **septate hyphae** confirms a dermatophyte (rules out Candida) ### Causative Organisms in Tinea Cruris | Organism | Frequency (Global/India) | Notes | |----------|--------------------------|-------| | **Trichophyton rubrum** | Most common overall (~50–60%) | Anthropophilic; most frequent cause worldwide | | **Epidermophyton floccosum** | Second most common (~20–30%) | Exclusively causes tinea cruris & tinea pedis; no hair invasion | | Trichophyton mentagrophytes | Less common in cruris | More common in tinea pedis/unguium | | Microsporum canis | Rare in groin | Zoophilic; primarily scalp/body | **Key Point:** *Epidermophyton floccosum* is classically highlighted as the **second most common** cause of tinea cruris and is the organism most specifically associated with the groin in standard dermatology and mycology textbooks (Fitzpatrick's Dermatology, Rippon's Medical Mycology). Among the options provided — where *T. rubrum* is absent — *E. floccosum* is the best answer, as *T. mentagrophytes* is primarily associated with tinea pedis and tinea unguium rather than tinea cruris. ### Why NOT the other options? - **Trichophyton mentagrophytes (A):** More commonly causes tinea pedis ("athlete's foot") and onychomycosis; less frequently implicated in tinea cruris compared to *E. floccosum*. - **Microsporum canis (B):** Zoophilic dermatophyte; primarily causes tinea capitis and tinea corporis; rarely causes tinea cruris. - **Candida albicans (D):** Produces **pseudohyphae and budding yeast** on KOH — not septate hyphae. Candidal intertrigo lacks the classic raised, scaly border with central clearing. ### KOH Mount Findings **High-Yield:** Septate hyphae on KOH preparation confirm a dermatophyte infection. *Epidermophyton floccosum* characteristically produces **club-shaped macroconidia** in clusters (2–4) and **no microconidia** — a distinguishing feature in culture. ### Pathophysiology 1. Dermatophyte colonizes stratum corneum via keratinolytic enzymes 2. Triggers inflammatory response → erythema and scaling 3. Central clearing due to host T-cell–mediated immunity **Clinical Pearl:** *Epidermophyton floccosum* is unique among dermatophytes in that it **does not infect hair** — it is restricted to skin and nails. This makes it a classic cause of tinea cruris and tinea pedis. (Reference: Fitzpatrick's Dermatology, 9th ed.; Rippon JW, Medical Mycology, 3rd ed.) ### Mnemonic: "FLOCK to the Groin" - **F**loccosum (*Epidermophyton*) — classic groin dermatophyte - **L**acks hair invasion - **O**nly skin and nails affected - **C**lub-shaped macroconidia in clusters - **K**OH shows septate hyphae