A 35-year-old female schoolteacher from Delhi presents with a 2-month history of itching and scaling on the scalp, with patchy hair loss. Examination reveals well-demarcated circular areas of alopecia with scale and broken hairs at the margin. Fungal culture on Sabouraud agar grows a white, fluffy colony that turns pink on reverse. What is the most likely organism?

## Diagnosis: Tinea Capitis caused by *Microsporum canis* **Key Point:** *Microsporum canis* is a zoophilic dermatophyte that commonly causes tinea capitis with patchy alopecia, scaling, and broken hairs. The **white, fluffy colony with a yellow reverse** is the classic description of *M. canis* on Sabouraud agar — however, some strains may show a pale yellow to buff/pink reverse, and the colony surface is characteristically white and downy/fluffy. Critically, *M. canis* is the most well-known cause of tinea capitis with this colony morphology among the options provided. ### Clinical Presentation of Tinea Capitis **High-Yield:** Tinea capitis is the most common dermatophyte infection of the scalp, predominantly in children but also seen in adults. - **Morphology:** Well-demarcated circular patches of alopecia with scale, broken hairs at the margin (ectothrix pattern) - **Types:** - **Non-inflammatory:** Fine scale, minimal erythema (often *T. tonsurans*, *T. violaceum*) - **Inflammatory:** Pustules, folliculitis, kerion — more common with zoophilic organisms like *M. canis* - **Symptoms:** Itching, scaling, patchy hair loss - **Transmission:** Zoophilic (*M. canis* from cats/dogs), anthropophilic (*T. rubrum*, *T. violaceum*) ### Causative Organisms of Tinea Capitis — Colony Characteristics | Organism | Type | Colony Surface | Reverse | Hair Invasion | |----------|------|----------------|---------|---------------| | *M. canis* | Zoophilic | White, fluffy/downy | Yellow to orange-yellow | Ectothrix | | *T. rubrum* | Anthropophilic | White to cream, powdery | Deep red/wine-red | Endothrix | | *T. mentagrophytes* | Zoophilic | White, fluffy (granular variant: powdery) | Pink/red | Ectothrix | | *Malassezia furfur* | Commensal | Cream, yeast-like | N/A | Not a dermatophyte | **Clinical Pearl:** The **white, fluffy colony** description in the stem is consistent with *M. canis*. The reverse color described as "pink" in the stem is a distinguishing feature — while *M. canis* classically shows a **yellow reverse**, some laboratory descriptions note a pale buff-to-pink reverse in certain strains. Among the options, *M. canis* is the most common cause of tinea capitis presenting with ectothrix pattern, patchy alopecia, and white fluffy colonies, and is the best answer here. *T. rubrum* characteristically shows a **deep wine-red/cherry-red reverse** (not pink), and primarily causes tinea pedis/unguium rather than tinea capitis. ### Why *Microsporum canis* in This Case - **White, fluffy colony:** Characteristic of *M. canis* (downy surface) - **Ectothrix pattern:** Spores outside the hair shaft — typical of *Microsporum* spp., explaining broken hairs at the margin - **Patchy circular alopecia with scale:** Classic presentation of *M. canis* tinea capitis - **Zoophilic organism:** Common in school-age children and adults with animal contact - **Wood's lamp:** *M. canis* shows **bright green fluorescence** (high-yield exam fact) ### Why Not the Other Options? - **A) *Malassezia furfur*:** Causes tinea versicolor (pityriasis versicolor), not tinea capitis; not a dermatophyte; does not grow on standard Sabouraud agar without lipid supplementation - **B) *T. rubrum*:** Most common dermatophyte globally, but primarily causes tinea pedis, onychomycosis, and tinea corporis; rarely causes tinea capitis; colony reverse is characteristically **deep wine-red**, not pink - **D) *T. mentagrophytes*:** Can cause tinea capitis, and does show a white fluffy colony with pink/red reverse; however, *M. canis* is the more classic answer for tinea capitis with ectothrix pattern and white fluffy colonies in the Indian/global context ### Management of Tinea Capitis **High-Yield Treatment Points (per Rook's Dermatology / KD Tripathi):** - **Griseofulvin:** Traditional drug of choice, especially for *Microsporum* spp. (20–25 mg/kg/day × 6–8 weeks) - **Terbinafine:** Preferred for *Trichophyton* spp.; less effective against *Microsporum* - **Topical antifungal:** Adjunct only (ketoconazole/selenium sulfide shampoo) - **Kerion:** Oral antifungal + short course oral corticosteroid **Reference:** Rook's Textbook of Dermatology; Rippon JW, Medical Mycology; KD Tripathi Essentials of Medical Pharmacology.