## Investigation of Choice for Tinea Cruris ### Why KOH Mount is Gold Standard **Key Point:** KOH mount preparation of scale is the most appropriate first-line confirmatory investigation for dermatophytosis, including tinea cruris. **High-Yield:** The KOH mount dissolves cellular material and reveals fungal elements (hyphae and spores) under light microscopy, providing rapid (15–30 minutes) and cost-effective confirmation. ### Procedure Essentials 1. Collect scale from the active border of the lesion (not the center) 2. Place on a glass slide with 10–20% KOH solution 3. Apply coverslip and gently heat (do not boil) 4. Examine under low power (×10) for branching septate hyphae ### Comparison of Diagnostic Methods | Investigation | Sensitivity | Specificity | Time to Result | Cost | Best Use | |---|---|---|---|---|---| | **KOH mount** | 60–80% | High | 15–30 min | Low | First-line screening | | **Fungal culture** | 90–95% | 100% | 2–4 weeks | Moderate | Species identification, treatment failure | | **Wood's lamp** | Variable | Low | Immediate | Minimal | Screening only (not diagnostic) | | **Histopathology** | High | High | 5–7 days | High | Atypical presentations | **Clinical Pearl:** A negative KOH mount does not exclude dermatophytosis; fungal culture should be sent if clinical suspicion is high and KOH is negative. **Mnemonic: SCALE** — **S**cale collection from border, **C**lean with KOH, **A**pply heat gently, **L**ook for hyphae, **E**xamine at ×10 magnification. ### Why KOH is Preferred Over Culture for Initial Diagnosis - Rapid turnaround allows prompt treatment initiation - Cost-effective for resource-limited settings - Sufficient for clinical management in most cases - Culture reserved for confirmation of species or treatment failure [cite:Park 26e Ch 13] 
Sign up free to access AI-powered MCQ practice with detailed explanations and adaptive learning.