A 32-year-old man presents with a 3-week history of pruritic, scaly patches on both feet, particularly in the interdigital spaces and soles. He works as a gym trainer and frequents the swimming pool. On examination, there is maceration and fissuring between the toes with erythema extending to the plantar surface. What is the most appropriate investigation to confirm the diagnosis of tinea pedis?

Explanation

## Investigation of Choice for Tinea Pedis ### Why KOH Mount is the Gold Standard **Key Point:** KOH preparation is the first-line, most cost-effective, and rapid confirmatory test for dermatophyte infections including tinea pedis. It dissolves cellular material and reveals fungal hyphae and spores within 10–15 minutes. ### Technique & Interpretation 1. **Sample collection:** Scrape scale from the active border (interdigital spaces, plantar surface, or nail if involved). 2. **Preparation:** Mix with 10% KOH solution on a glass slide. 3. **Microscopy:** Examine under low power (×10) for septate hyphae and spores. 4. **Sensitivity:** 50–70% for tinea pedis; higher if multiple scrapings taken. ### Why This Case Fits - **Interdigital tinea pedis** (athlete's foot) is the most common presentation. - Maceration, fissuring, and erythema are classic signs. - KOH mount provides rapid, bedside confirmation without waiting for culture. **High-Yield:** KOH mount is the **initial diagnostic test**; fungal culture is reserved for treatment failures, atypical presentations, or when species identification is needed (e.g., non-dermatophyte molds). **Clinical Pearl:** In tinea pedis, always examine the fourth and fifth interdigital spaces first—this is where infection typically begins. ### Comparison of Diagnostic Methods | Investigation | Sensitivity | Specificity | Time | Cost | Best Use | |---|---|---|---|---|---| | KOH mount | 50–70% | High | 10–15 min | Low | First-line confirmation | | Fungal culture | 80–90% | Very high | 2–4 weeks | Moderate | Species ID, treatment failure | | Wood's lamp | N/A | N/A | Immediate | Low | Screening only (not reliable for most tinea) | | Histopathology | Very high | Very high | 1 week | High | Diagnostic uncertainty, biopsy needed | [cite:Fitzpatrick's Dermatology 10e Ch 134]