Mucocutaneous fungal infections are caused by dermatophytes (Microsporum, Epidermophyton, and Trichophyton) and yeasts. About 40 species in the three dermatophyte genera can cause tinea pedis and manus, tinea capitis, tinea corpo-ris, tinea cruris, and onychomycosis. Yeasts of Candida can cause diaper dermatitis, balanitis, vulvovaginitis, and thrush (Fig. 33-40). The yeastlike organism of Malassezia (Pityros-porum) causes tinea versicolor and contributes to seborrhea. Although tinea versicolor has the name tinea in it, it is not a true dermatophyte.
The most important test for a suspected fungal infection is the KOH prep. Scrape the leading edge of the lesion onto a slide using the side of a #15 scalpel or another slide. Use the coverslip to push the scale into the center of the slide. Add 2 drops of KOH (with or without fungal stain) to the slide, and place coverslip on top. Examine with microscope starting with 10x and low light to look for the cells and hyphae. The fungal stain helps the hyphae to stand out among the epithelial cells. Look for groups of cells that appear to have fungal elements within them; don't be fooled by cell borders that look linear and branching. Switch to 40x to confirm any areas that appear to have fungal elements by looking for true fungal morphology. The fungal stains bring out these characteristics, including cell walls, nuclei, and arthroconidia (Fig. 33-41). KOH test characteristics without fungal stain are sensitivity, 77% to 88%, and specificity, 62% to 95% (Thomas, 2003).
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