In tinea capitis caused by Microsporum canis, the type of hair invasion and fluorescence pattern are:
- A Endothrix invasion; no fluorescence under Wood's lamp
- B Ectothrix invasion; bright green fluorescence under Wood's lamp ✓
- C Favic invasion (scutula); white fluorescence
- D Endothrix invasion; dull green fluorescence
Correct answer: B. Ectothrix invasion; bright green fluorescence under Wood's lamp
Explanation
Microsporum canis (and M. audouinii) cause ectothrix invasion — arthroconidia form a sheath around the outside of the hair shaft without invading it internally. These fluoresce a characteristic bright greenish-yellow under Wood's lamp due to pteridine metabolites (tryptophan derivatives). Endothrix species (Trichophyton tonsurans, T. violaceum) invade inside the hair shaft and do NOT fluoresce. Favus (T. schoenleinii) shows pale green fluorescence and characteristic mousy odour.
Reference: Neena Khanna Illustrated Synopsis of Dermatology & STD, 6th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.