A 28-year-old woman with tinea capitis (scalp ringworm) presents with patchy alopecia and scale. Clinical examination and KOH mount are inconclusive. Which investigation is most appropriate to confirm the diagnosis and determine whether the infection is endothrix or ectothrix?

## Diagnosis and Classification of Tinea Capitis **Key Point:** Fungal culture with microscopic examination of spore arrangement is the only investigation that both confirms tinea capitis AND distinguishes endothrix from ectothrix morphology—critical for prognosis and treatment decisions. ### Endothrix vs. Ectothrix Classification | Feature | Endothrix | Ectothrix | |---------|-----------|----------| | **Spore location** | Inside hair shaft | Outside hair shaft | | **Common organisms** | *T. tonsurans*, *T. violaceum* | *T. mentagrophytes*, *M. canis*, *M. audouinii* | | **Geographic prevalence** | More common in India, Africa | More common in Europe, Americas | | **Prognosis** | Often self-limiting by puberty | May require systemic therapy | | **Microscopy finding** | Hyphae and spores within cortex | Chains of spores around hair shaft | **High-Yield:** In India, *Trichophyton tonsurans* (endothrix) is increasingly common in tinea capitis. Identifying endothrix vs. ectothrix guides whether systemic antifungals are needed or topical therapy may suffice. ### Why Culture with Spore Morphology Matters **Clinical Pearl:** KOH mount may show fungal elements but cannot reliably differentiate spore arrangement. Only cultured isolates examined under light microscopy (or lactophenol cotton blue staining) reveal whether spores are arranged inside (endothrix) or outside (ectothrix) the hair shaft. **Mnemonic: ENDO-INSIDE, ECTO-OUTSIDE** — **ENDO**thrix spores are **INSIDE** the hair; **ECTO**thrix spores are **OUTSIDE**. [cite:Park 26e Ch 8; Fitzpatrick 9e Ch 32]