A 28-year-old woman from Mumbai presents with a 6-week history of pruritic, scaly patches on her trunk and proximal limbs. Examination reveals well-demarcated, erythematous plaques with central clearing and a raised, active border. The lesions are most prominent in skin fold areas. KOH mount of scale shows septate hyphae. She reports recent contact with a stray dog. What is the most likely causative organism?

Explanation

## Clinical Diagnosis: Tinea Corporis from Microsporum canis ### Key Clinical Features **Key Point:** Microsporum canis is a zoophilic dermatophyte transmitted from animals (dogs, cats) to humans, causing tinea corporis with characteristic central clearing and active borders. ### Organism Identification | Feature | Microsporum canis | Trichophyton rubrum | Epidermophyton floccosum | |---------|-------------------|-------------------|-------------------------| | **Ecology** | Zoophilic (animals) | Anthropophilic | Anthropophilic | | **Source** | Dog, cat contact | Human-to-human | Human-to-human | | **KOH mount** | Septate hyphae, spores | Septate hyphae | Septate hyphae | | **Culture** | Fluffy, white colonies | Flat, buff/tan colonies | Flat, tan colonies | | **Wood's lamp** | Blue-green fluorescence | Non-fluorescent | Non-fluorescent | | **Common sites** | Face, trunk, limbs | Palms, soles, nails | Groin, axillae | ### High-Yield Clinical Pearls **High-Yield:** The history of recent contact with a stray dog is the critical epidemiologic clue pointing to a zoophilic organism. Microsporum canis is the most common zoophilic dermatophyte in India. **Clinical Pearl:** Microsporum canis produces a characteristic **blue-green fluorescence under Wood's lamp** — a useful bedside diagnostic aid. This feature, combined with animal exposure history, makes the diagnosis highly likely. **Mnemonic:** **ZOO = Zoophilic Organisms** — Microsporum canis, M. audouinii, Trichophyton verrucosum are transmitted from animals. ### Pathophysiology Zoophilic dermatophytes cause more inflammatory lesions (erythema, scaling, central clearing) compared to anthropophilic species because the human immune system recognizes them as "foreign." This explains the active border and pruritus in this case. ### Diagnostic Confirmation 1. **KOH mount:** Septate hyphae confirm fungal infection (already positive in this case) 2. **Culture on Sabouraud dextrose agar:** Fluffy white colonies with rapid growth 3. **Wood's lamp:** Blue-green fluorescence (positive in ~50% of M. canis cases) 4. **Microscopy:** Conidiospores arranged in chains [cite:Fitzpatrick's Dermatology 10e Ch 133]