A 32-year-old man presents with pruritic, scaly patches on the groin and inner thighs for 3 weeks. Examination reveals well-demarcated erythematous plaques with central clearing and an active advancing border. KOH mount of scale shows septate hyphae. Which investigation is most appropriate to identify the causative organism and guide antifungal therapy?

Question

Accepted Answer

D. Fungal culture on Sabouraud dextrose agar. ## Investigation of Choice for Dermatophytosis Confirmation

**Key Point:** While KOH mount confirms the presence of fungal elements (septate hyphae), fungal culture on Sabouraud dextrose agar is the gold standard for species identification and susceptibility testing.

### Why Fungal Culture is Essential

| Aspect | KOH Mount | Fungal Culture |
|--------|-----------|----------------|
| **Confirms fungal infection** | Yes | Yes |
| **Species identification** | No | Yes |
| **Antifungal susceptibility** | No | Yes |
| **Turnaround time** | Minutes | 2–4 weeks |
| **Clinical utility** | Rapid screening | Definitive diagnosis |

**High-Yield:** Tinea cruris (jock itch) is most commonly caused by *Trichophyton rubrum* or *T. mentagrophytes*. Culture identifies the exact species, which helps predict treatment response and rule out non-dermatophyte molds (which may require different therapy).

### Clinical Pearl

KOH mount is the rapid bedside test that confirms fungal infection and guides empiric antifungal therapy initiation. However, for:
- Recurrent or treatment-resistant cases
- Suspected non-dermatophyte infection
- Epidemiological tracking

...fungal culture on Sabouraud dextrose agar (with or without antibiotics to suppress bacterial overgrowth) is mandatory.

**Mnemonic: SAB-CULTURE** — **S**abouraud agar for **A**ccurate **B**iological identification of **CULTURE**d dermatophytes.

[cite:Park 26e Ch 8]

![Dermatophytosis — Tinea diagram](https://mmcphlazjonnzmdysowq.supabase.co/storage/v1/object/public/blog-images/explanation/3986.webp)

Answer

A. Bacterial culture on blood agar

Answer

B. Tzanck smear

Answer

C. Gram stain of the lesion

Explanation

Investigation of Choice for Dermatophytosis Confirmation

Why Fungal Culture is Essential

Clinical Pearl

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Aspect	KOH Mount	Fungal Culture
Confirms fungal infection	Yes	Yes
Species identification	No	Yes
Antifungal susceptibility	No	Yes
Turnaround time	Minutes	2–4 weeks
Clinical utility	Rapid screening	Definitive diagnosis