Question

Black dot ringworm is caused by

• A. Microsporum
• B. Trichophyton
• C. Epidermophyton
• D. Candida

Hint:

Consider which fungus grows inside the hair shaft rather than on its surface.

Answer 1

The Correct Option is B

Step 1: Understanding the Question:
The question asks which fungal genus causes the black dot pattern of ringworm, a form of tinea capitis.

Step 2: Key Concept or Approach:
Black dot ringworm is a form of tinea capitis in which infected hairs break off at the level of the scalp, leaving visible black dots formed by the retained hair stubs plugged with fungal spores. This pattern occurs with endothrix infections, where the fungus invades and fills the hair shaft from within, weakening it so it snaps at the surface.

Step 3: Working Through the Options:
Microsporum species typically produce an ectothrix pattern, coating the outside of the hair shaft, and are associated with fluorescence under Wood's lamp rather than the black dot pattern. Trichophyton species, particularly Trichophyton tonsurans and Trichophyton violaceum, are classic endothrix organisms that invade within the hair shaft and cause hairs to break at the scalp surface, producing the black dot appearance. Epidermophyton does not infect hair at all; it is limited to skin and nails. Candida causes mucocutaneous infections and is not associated with this pattern of scalp ringworm.

Step 4: Conclusion:
The correct answer is Trichophyton, the endothrix organism responsible for black dot ringworm.