A 15-year-old boy presents to his physician with several weeks of slowly worsening pruritus of both of his feet. Heis otherwise well and taking no medications. On examination, he has bilateral, erythematous, dry scaling lesionsthat are most obvious in the interdigital web spaces and on the soles. There is no bleeding or exudate. Whatwould most likely be found in a potassium hydroxide (KOH) mount of a scraping of the affected skin?

Branching, septate hyphae

The correct answer is B. This is a case of tinea pedis, or athletes foot, caused by a variety of dermatophytic fungi, which are easily detected on alkali mounts of scraped skin as colorless, branching hyphae with cross-walls. Hyphae with rosettes of conidia (choice A) describes the environmental/transmission form for Sporothrix schenckii, the agent of rose gardeners disease, which is a subcutaneous mycosis. Budding yeasts (choice C) describes the form found in clinical specimens from patients with sporotrichosis. Hyphae, arthroconidia, and blastoconidia (choice D) would be found in clinical specimens from patients infected with Trichosporon beigelii (white piedra), which is a superficial mycosis of the hair of the head. Pigmented, septate hyphal fragments (choice E) would be found in cases of phaeohyphomycosis, a diverse group of cyst-forming subcutaneous, pigmented (dematiaceous) fungi, rare in the U.S. Short, curved hyphae and round yeasts (choice F), or the spaghetti and meatball presentation in clinical specimens, are characteristic of Malassezia furfur, the agent of pityriasis versicolor.