A 35-year-old woman from rural Maharashtra presents with a 2-month history of progressive hair loss from the scalp. Examination reveals a well-demarcated patch of alopecia with scaling, erythema, and broken hairs (not pulled out at the root). Some hairs appear black and dotted (black dot ringworm). Fungal culture on Sabouraud medium shows a slow-growing, buff-coloured colony with a red pigment on the reverse side. What is the most likely diagnosis and organism?

Explanation

## Clinical Diagnosis: Tinea Capitis (Scalp Ringworm) ### Key Clinical Features **Key Point:** Tinea capitis is a dermatophyte infection of the scalp and hair shaft, most common in children but can occur in adults, especially in tropical regions like India. The clinical presentation strongly suggests tinea capitis: - **Well-demarcated patch of alopecia** with scaling and erythema - **Broken hairs** (not pulled out at the root) — characteristic of dermatophyte invasion of the hair shaft - **Black dot ringworm** — broken hairs that appear as black dots on the scalp; indicates endothrix infection (fungus within the hair shaft) - Progressive course over 2 months - Rural setting (higher exposure to infected animals or contaminated fomites) ### Culture Findings: Trichophyton rubrum **High-Yield:** The culture description is classic for *T. rubrum*: - **Slow-growing** colony (2–3 weeks on Sabouraud dextrose agar) - **Buff or cream-coloured** on the obverse (top) surface - **Red/reddish-brown pigment on the reverse** (bottom) side — this is pathognomonic for *T. rubrum* **Clinical Pearl:** The red pigment on the reverse is due to production of a diffusible pigment (likely a quinone derivative) and is the single most useful colony characteristic for identifying *T. rubrum* in culture. ### Dermatophytes Causing Tinea Capitis: Comparison | Organism | Type | Prevalence in India | Culture Features | Clinical Pattern | |----------|------|-------------------|------------------|------------------| | **Trichophyton rubrum** | Endothrix | Most common (>50%) | Slow; buff obverse; **red reverse** | Black dot ringworm; non-inflammatory | | Microsporum canis | Ectothrix | Rare in India | Fast; white/grey; no red reverse | Inflammatory; zoophilic (animals) | | Trichophyton mentagrophytes | Ectothrix | Moderate | Fast; white; no red reverse | Inflammatory; can be zoophilic | | Microsporum audouinii | Ectothrix | Rare; declining | Slow; grey-white | Non-inflammatory; anthropophilic | **Mnemonic:** **RED REVERSE = T. RUBRUM** — the red pigment on the reverse side of the colony is the diagnostic hallmark. ### Endothrix vs. Ectothrix Infection | Feature | Endothrix (T. rubrum) | Ectothrix (M. canis, T. mentagrophytes) | |---------|----------------------|----------------------------------------| | Fungus location | **Inside** the hair shaft | **Outside** the hair shaft | | Hair appearance | Black dots (broken hairs) | Halo of spores around hair | | Inflammation | Usually mild | Often more inflammatory | | Wood's lamp | Usually negative | May show blue-green fluorescence (M. canis) | | Culture growth | Slow | Fast | **Key Point:** Black dot ringworm is pathognomonic for endothrix infection, and *T. rubrum* is the most common endothrix dermatophyte in India. ### Pathophysiology of Hair Invasion ```mermaid flowchart TD A[Dermatophyte spores contact scalp]:::outcome --> B[Fungus invades stratum corneum]:::action B --> C{Infection pattern?}:::decision C -->|Endothrix| D[Fungus grows inside hair shaft]:::action C -->|Ectothrix| E[Fungus grows outside hair shaft]:::action D --> F[Hair becomes brittle & breaks]:::action E --> G[Spore sheath around hair]:::action F --> H[Black dot appearance on scalp]:::outcome E --> I[Halo of inflammation around hair]:::outcome H --> J[Diagnosis: Tinea capitis endothrix]:::outcome I --> K[Diagnosis: Tinea capitis ectothrix]:::outcome ``` ### Microscopy and Culture - **KOH mount:** Shows hyphae and spores within/around hair shaft (depending on endothrix vs. ectothrix) - **Culture on Sabouraud dextrose agar:** Incubate at 25–28°C for 2–4 weeks - *T. rubrum*: Slow growth, buff obverse, **red reverse** (diagnostic) - *M. canis*: Fast growth, white/grey, no red reverse; may fluoresce blue-green under Wood's lamp - **Wood's lamp:** Usually negative for *T. rubrum*; may be positive for *M. canis* ### Treatment **High-Yield:** Tinea capitis requires **systemic antifungal therapy** because topical agents cannot penetrate the hair shaft effectively. - **First-line:** Griseofulvin 20–25 mg/kg/day for 6–8 weeks (microsize formulation) - **Alternative:** Terbinafine 125–250 mg daily for 4–6 weeks (faster; preferred in some guidelines) - **Adjunct:** Antifungal shampoo (ketoconazole, selenium sulfide) 2–3 times weekly to reduce spore shedding **Clinical Pearl:** Griseofulvin is still widely used in India due to cost and established efficacy, though terbinafine is faster and equally effective.