A 48-year-old barefoot farmer from coastal Karnataka has a slowly enlarging, warty, cauliflower-like plaque on his left foot that began years ago after a thorn prick. The surface shows scattered black dots. A KOH mount of scrapings from the black dots shows thick-walled, golden-brown, rounded cells with septa in two planes, resembling copper pennies. Which organism is the most likely cause?
Answer: B. Fonsecaea pedrosoi. A chronic verrucous lesion after traumatic implantation, with brown, thick-walled, muriform cells (sclerotic or Medlar bodies, "copper pennies"), is diagnostic of chromoblastomycosis. Fonsecaea pedrosoi is the commonest agent worldwide and in India; others include Cladophialophora carrionii and Phialophora verrucosa. These dematiaceous fungi grow as slow, dark, velvety colonies on Sabouraud dextrose agar. Histology typically shows pseudoepitheliomatous hyperplasia with microabscesses containing the sclerotic bodies.
A. Sporothrix schenckii causes nodules spreading along lymphatics; tissue shows sparse cigar-shaped yeasts and asteroid bodies, not pigmented muriform cells.
C. Madurella mycetomatis causes eumycetoma, with tumefaction, sinuses and black grains made of hyphae, rather than single sclerotic bodies.
D. Blastomyces dermatitidis can produce verrucous skin lesions, but in tissue it is a hyaline, thick-walled yeast with broad-based budding and no brown pigment.
