A 28-year-old man with a 6-week history of tinea pedis (athlete's foot) affecting the interdigital spaces presents with treatment failure despite 4 weeks of topical terbinafine. KOH mount performed 2 weeks ago was positive for fungal elements. What is the most appropriate next investigation to guide further management?

## Investigation of Choice for Treatment-Resistant Tinea ### Clinical Context: Treatment Failure **Key Point:** When a dermatophyte infection fails to respond to appropriate topical therapy after 4 weeks, fungal culture with sensitivity testing becomes the investigation of choice to identify the causative organism and guide targeted therapy. **High-Yield:** Treatment failure in tinea may result from: - Non-adherence to therapy - Incorrect diagnosis (non-dermatophyte infection, e.g., *Candida*, *Scytalidium*) - Antifungal resistance (rare but documented) - Inadequate drug penetration or duration ### Why Culture & Sensitivity is Essential Here ```mermaid flowchart TD A[Clinically suspected tinea]:::outcome --> B[KOH mount positive]:::action B --> C[Start topical antifungal]:::action C --> D{Response after 4 weeks?}:::decision D -->|Yes| E[Continue therapy, clinical cure]:::outcome D -->|No| F[Send fungal culture + sensitivity]:::action F --> G{Culture result}:::decision G -->|Dermatophyte, sensitive| H[Improve adherence, switch to systemic agent]:::action G -->|Non-dermatophyte or resistant| I[Identify organism, adjust therapy]:::action H --> J[Clinical cure]:::outcome I --> J ``` ### Diagnostic Algorithm for Treatment Failure | Step | Investigation | Rationale | |---|---|---| | 1 | Repeat clinical assessment | Confirm diagnosis, assess adherence | | 2 | Fungal culture + sensitivity | Identify organism, detect resistance | | 3 | Systemic antifungal (if dermatophyte confirmed) | Improve bioavailability for refractory cases | **Clinical Pearl:** *Trichophyton rubrum* and *T. mentagrophytes* are the most common dermatophytes in tinea pedis. Resistance to terbinafine is rare but documented; culture identifies non-dermatophyte mimickers (e.g., *Candida*, *Scytalidium dimidiatum*) that require different therapy. **Mnemonic: CULTURE** — **C**onfirm organism, **U**nderstand sensitivity, **L**ocate resistance, **T**arget therapy, **U**pgrade to systemic, **R**esolve infection, **E**nsure cure. ### Why Other Investigations Are Suboptimal Here - **Repeat KOH mount:** Already confirmed fungal infection; does not identify organism or sensitivity - **Histopathology with PAS:** Useful for atypical presentations or diagnosis of onychomycosis; overkill for tinea pedis and does not provide sensitivity data - **Molecular PCR:** Rapid species identification but does not assess antifungal susceptibility; not standard first-line in most settings [cite:Park 26e Ch 13]