A 32-year-old male presents with pruritic, scaly patches on the groin and inner thighs for 3 weeks. Clinical examination reveals well-demarcated erythematous plaques with a raised border and central clearing. The lesions spare the scrotum. Which investigation is most appropriate to confirm the diagnosis of tinea cruris?

## Diagnosis of Tinea Cruris **Key Point:** KOH (potassium hydroxide) mount preparation of scale from the active border is the gold standard for confirming dermatophyte infection in tinea cruris. ### Why KOH Mount is the Investigation of Choice 1. **Direct visualization of fungal elements**: KOH dissolves cellular material and keratin, allowing clear visualization of branching hyphae and spores under light microscopy. 2. **High specificity for dermatophytes**: The characteristic appearance of septate hyphae with acute-angle branching is diagnostic. 3. **Rapid turnaround**: Results available within 24 hours, faster than culture. 4. **Cost-effective**: Inexpensive and widely available in resource-limited settings. ### Specimen Collection Technique **High-Yield:** Always scrape from the **active border** (periphery) of the lesion, not the center. The center may show resolution with fewer organisms, while the advancing edge has maximum fungal load. ### Microscopic Findings in Tinea Cruris | Feature | Finding | |---------|----------| | Hyphae | Septate, branching at acute angles | | Spores | Round to oval, arranged in chains | | Appearance | "Spaghetti and meatballs" pattern | | Causative organisms | *Trichophyton rubrum* (most common), *T. mentagrophytes* | **Clinical Pearl:** The characteristic sparing of the scrotum in tinea cruris (due to high moisture and lack of keratinous substrate) helps differentiate it from candidiasis, which typically involves the scrotum. ### Differential Considerations - ~~Tzanck smear~~ is used for viral infections (HSV, VZV), not fungal infections. - ~~Wood's lamp~~ has limited utility in tinea; it is useful for *Microsporum canis* (blue-green fluorescence) and *Microsporum audouinii*, but most tinea species do not fluoresce. - ~~Bacterial culture~~ is inappropriate; tinea cruris is a fungal, not bacterial, infection. **Mnemonic:** **KOH for Keratin Organisms in Hyphal form** — KOH dissolves keratin to reveal hyphae.