tinea cruris

**Definition:** Superficial fungal (dermatophyte) infection of the groin, pubic region, perineum, and proximal medial thighs. It is a type of tinea corporis occurring in intertriginous areas.

**Etiology:** The most common causes are Trichophyton rubrum (most common worldwide) and Epidermophyton floccosum. Trichophyton mentagrophytes (interdigitale) is a less common cause. Dermatophytes are keratinophilic. They invade only the stratum corneum, hair, and nails, and do not invade living tissue.

**Risk factors/epidemiology:** Male sex (adolescent and adult men), obesity, diabetes mellitus, immunosuppression, hot/humid climate, occlusive or tight clothing, excessive sweating, and shared towels or athletic equipment. Concomitant tinea pedis is a common source of autoinoculation (fungus spreads from the feet to the groin via underwear or towels). Treating the feet too reduces recurrence.

**Pathophysiology:** Fungal arthroconidia adhere to keratinocytes in warm, moist, macerated skin. The fungus then secretes keratinases and proteases to digest keratin. The inflammatory response (type IV hypersensitivity) produces the erythematous, scaly border.

**Clinical features:** - Pruritic, erythematous, well-demarcated plaques with an advancing, raised, scaly (sometimes vesicular or pustular) border and central clearing (annular, "ring" morphology). - Begins in the crural fold and spreads to the inner thighs and sometimes the buttocks and pubic area. - **The scrotum/penis are typically spared**, which helps distinguish it from cutaneous candidiasis, which involves the scrotum and often has satellite pustules or papules. - Chronic scratching can lead to lichenification and hyperpigmentation. Topical steroids may produce tinea incognito, with an atypical appearance.

**Diagnosis:** Clinical, confirmed by a KOH preparation of skin scrapings from the active border. It shows **septate, branching hyphae** (candida shows pseudohyphae and budding yeast). Fungal culture on Sabouraud agar can be done if the diagnosis is unclear. Wood's lamp is generally not useful for tinea cruris.

**Differential diagnosis:** - **Erythrasma:** Corynebacterium minutissimum. It produces coral-red fluorescence under Wood's lamp. The lesions are brown, flat, and non-scaly, and KOH is negative. Treated with erythromycin. - **Candidal intertrigo:** Beefy red with satellite lesions, involves the scrotum. - **Inverse psoriasis:** Smooth, shiny, well-demarcated plaques without scale or central clearing. - **Seborrheic dermatitis, contact dermatitis, and intertrigo.** - **Extramammary Paget disease:** Persistent, unresponsive eczematous plaque.

**Treatment:** - First-line: topical antifungals for 2–4 weeks. Options are terbinafine (an allylamine, which inhibits squalene epoxidase), or azoles such as clotrimazole, miconazole, and ketoconazole (which inhibit 14-α-demethylase). - Oral therapy is for extensive, recalcitrant, or recurrent disease, or for immunocompromised patients. Options include terbinafine, itraconazole, and fluconazole. Oral griseofulvin is less effective and rarely used. - Keep the area dry and wear loose clothing. Treat tinea pedis simultaneously, and avoid sharing towels. - Avoid combination antifungal-steroid creams.

**High-yield boards pearls:** - Ring-shaped lesion with central clearing, raised scaly border, and **scrotal sparing** → tinea cruris. - KOH: septate hyphae. Candida: pseudohyphae and yeast. - Tinea cruris, corporis, and pedis are caused by *Trichophyton*, *Microsporum*, and *Epidermophyton*. *Epidermophyton* infects skin and nails (not hair); *Microsporum* infects skin and hair (not nails); *Trichophyton* infects skin, hair, and nails. - Erythrasma (coral-red fluorescence on Wood's lamp) is a classic mimic.