A patient with extensive tinea capitis shows greenish fluorescence under Wood's lamp. Which dermatophyte species is most likely responsible, and what type of hair invasion does it cause?

• A Trichophyton violaceum — endothrix invasion
• B Trichophyton tonsurans — endothrix, does not fluoresce
• C Epidermophyton floccosum — does not infect hair
• D Microsporum canis — ectothrix invasion with small-spore sheath ✓

Explanation

Greenish fluorescence (bright yellow-green) under Wood's lamp in tinea capitis is characteristic of Microsporum species (M. canis, M. audouinii). These cause ectothrix infection — fungal spores form a sheath around the hair shaft (outside the hair), and ectothrix species typically fluoresce. Trichophyton species (T. tonsurans, T. violaceum) cause endothrix invasion (spores within the hair shaft) and do NOT fluoresce. Epidermophyton floccosum only infects skin and nails, not hair. T. schoenleinii causes favus and shows dull grey-green fluorescence.

Reference: Neena Khanna Illustrated Synopsis of Dermatology & STD, 6th ed.

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Written and medically reviewed by the StethoPrep medical team.