A young man notices firm black nodules attached to the shafts of his scalp hairs along with mild scaling. KOH mount of a crushed nodule shows a cluster of dark brown, thick-walled ascospores arranged in a gelatinous matrix. Which organism is the cause and how is it distinguished from the analogous white form?

• A Hortaea werneckii, distinguished from Cladosporium by dematiaceous hyphae
• B Malassezia furfur, distinguished from Trichosporon by lipid dependence
• C Trichophyton rubrum, distinguished from Epidermophyton floccosum by microconidia
• D Piedraia hortae, distinguished from Trichosporon beigelii by dark colour and ascospore production ✓

Explanation

Black piedra is caused by Piedraia hortae, an ascomycete forming hard dark nodules on hair shafts that contain asci with crescentic ascospores. White piedra is caused by the basidiomycete Trichosporon beigelii and produces softer, lighter nodules composed of arthroconidia and blastoconidia without ascospores. Both are superficial infections limited to the hair shaft with no tissue invasion. Malassezia causes pityriasis versicolor, Hortaea causes tinea nigra of the palm, and dermatophytes invade keratinised tissue rather than forming nodules.

Reference: Jagdish Chander, Textbook of Medical Mycology, 4th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.