Which of the following is the most reliable diagnostic method for confirming dermatophytosis in tinea corporis?

Explanation

## Diagnostic Methods in Dermatophytosis **Key Point:** KOH mount is the gold standard rapid diagnostic method for confirming dermatophytosis. It dissolves cellular material and reveals fungal elements (hyphae and spores) under light microscopy. ### Diagnostic Approach to Tinea Corporis ```mermaid flowchart TD A[Suspected Tinea Corporis]:::outcome --> B{Confirm diagnosis?}:::decision B -->|Rapid confirmation| C[KOH Mount]:::action C --> D[Hyphae + Spores visible]:::outcome B -->|Culture for species ID| E[Sabouraud Dextrose Agar]:::action E --> F[Identify dermatophyte species]:::outcome B -->|Avoid| G[Gram stain - not useful]:::urgent B -->|Avoid| H[Tzanck smear - for HSV/VZV]:::urgent ``` ### Diagnostic Methods Comparison | Method | Specimen | Preparation | Findings | Sensitivity | Speed | Use | |--------|----------|-------------|----------|-------------|-------|-----| | **KOH Mount** | Scales, nail debris | 10-40% KOH + heat | Septate hyphae, spores | 60–80% | 15–30 min | **Gold standard** | | **Culture** | Scales, hair, nails | Sabouraud medium | Colony morphology, species ID | 85–95% | 2–4 weeks | Confirmation + species ID | | **Wood's lamp** | Direct visualization | UV 365 nm | Variable fluorescence | Low | Immediate | Screening only | | **Gram stain** | Skin scrapings | Gram reagent | Bacteria, not fungi | Not applicable | 10 min | **Not useful for fungi** | | **Tzanck smear** | Vesicular fluid | Methylene blue | Multinucleate cells | — | 10 min | **HSV/VZV only** | **High-Yield:** KOH dissolves the cell membrane and cytoplasm, leaving behind the fungal cell wall (chitin) intact. This makes hyphae and spores clearly visible as refractile structures. ### Technique for KOH Mount 1. **Specimen collection:** Scrape the advancing edge of the lesion with a sterile blade or curette 2. **Mounting:** Place scrapings on a glass slide 3. **KOH addition:** Add 10–40% KOH solution 4. **Heating:** Gently heat (do not boil) to accelerate clearing 5. **Microscopy:** Examine under low power (×10) for branching septate hyphae and spores **Clinical Pearl:** Always scrape the *active border* of the lesion, not the center. The center may be healing and contain fewer organisms. **Mnemonic:** **KOH CLEARS** — KOH, Clearing agent, Hyphae visible, Yeast forms, Rapid, Economical, Accessible, Reliable, Scales/nail debris.