## Microsporum canis vs. Trichophyton tonsurans in Tinea Capitis ### Comparative Features | Feature | Microsporum canis | Trichophyton tonsurans | |---------|---|---| | **Wood's Lamp** | Bright blue-green fluorescence | No fluorescence (non-fluorescent) | | **Microscopy** | Spores outside hair shaft (ectothrix) | Spores inside hair shaft (endothrix) | | **Inflammation** | Moderate to severe | Mild to moderate | | **Alopecia** | Patchy, well-demarcated | Diffuse, "black dot" appearance | | **Pustules** | Present | Rare | | **Prevalence** | Less common in India | More common in India | ### Key Distinguishing Feature **Key Point:** **Microsporum canis fluoresces bright blue-green under Wood's lamp**, whereas **Trichophyton tonsurans does NOT fluoresce**. This is the single most useful clinical discriminator for rapid differentiation. **High-Yield:** M. canis = **Fluorescent** (remember: "M for Microsporum, M for Magnificent glow"). T. tonsurans = **Non-fluorescent**. This distinction is tested frequently in NEET PG. **Mnemonic:** **MCAT** — **M**icrosporum **C**anis **A**lways **T**ransluminates (fluoresces). **Clinical Pearl:** In India, T. tonsurans is the most common cause of tinea capitis, while M. canis is less frequent. However, when M. canis is encountered, the Wood's lamp finding is diagnostic and avoids the need for culture confirmation. ### Why Other Features Are Not Discriminatory **Branching septate hyphae:** Both organisms show this on KOH mount. The difference is in the **location** of spores (ectothrix vs. endothrix), not the presence of hyphae. **Inflammation and pustules:** While M. canis typically causes more inflammation, this is variable and overlaps with T. tonsurans presentations. Not a reliable discriminator in all cases. 
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