A 6-year-old boy has several small round patches of scaling alopecia on the scalp studded with black dots representing broken-off hairs. Potassium hydroxide examination shows endothrix arthrospores. The most likely causative organism is:
- A Microsporum canis
- B Trichophyton rubrum
- C Trichophyton tonsurans ✓
- D Epidermophyton floccosum
Correct answer: C. Trichophyton tonsurans
Explanation
Black dot tinea capitis results from endothrix invasion in which hyphae fill the hair shaft, causing it to break at the scalp surface and leave black dots. Trichophyton tonsurans is the classic endothrix cause and is now the commonest agent of tinea capitis in many regions. Microsporum canis produces ectothrix spores and green fluorescence under Wood lamp, while Epidermophyton floccosum does not invade hair.
Reference: Rook's Textbook of Dermatology, 9th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.