An 18-year-old white male high-school student visits the family physician complaining of a diffuse, painful rash extending from his midthigh to his navel region. In recounting the history of the rash, he indicates that one of his football teammates gave him topical hydrocortisone to treat a minor groin rash. A KOH scraping of the lesion reveals hyaline hyphae and a portion of the scraping is submitted for culture. A schematic of the microscopic observation of the culture is shown below. The most likely etiology is
The presence of hyaline hyphae supports a diagnosis of a dermatophyte infection (see the table below). Dermatophyte infections may be exacerbated by the use of topical steroids. With such exposure the infection may not give the typical picture and may be called tinea incognito. This infection started in the groin region where such infections are known as tinea cruris or jock itch. Dermatophytes are identified in culture by the macroscopic morphology of the colony and the microscopic morphology of asexual production of conidia. Both macroconidia and microconidia may be produced but one is more abundant and used in identification while the other is rare. Trichophyton species produce numerous microconidia whose appearance and location are useful in their identification. Microsporum species (except M. audouinii) produce macroconidia that are borne singly and that have distinctive shapes and rough walls. Epidermophytonfloccosum produces only macroconidia. These are usually produced in clusters and have a distinctive clavate shape sometimes described as "beaver-tail". The schematic shows clavate macroconidia with this shape, which is consistent with E. floccosum.SELECTED ANTIFUNGAL DRUGS, MODE OF ACTION AND MECHANISMS OF RESISTANCEAntifungal AgentRouteMode of ActionMechanism(s) of ResistanceTolnaftate TopicalInhibition of squalene epoxidase. (enzyme in the ergosterol biosynthesis pathway)Resistant isolates very rare; likely mechanisms include efflux pumps, mutation, or over expression target, nonspecific stress adaptationAllylamines NaftifineTopicalTerbinafineOral, topicalFlucytosine (5-flurocytosine)OralInhibition of DNA and RNA synthesisChange in permease, cytosine deaminase, or uracil phosphoribosyl transferase activityEchinocandins AnidalufunginIVInhibition of fungal cell wall glucan synthesisMutation in subunit b-l,3-glucan synthaseCaspofunginMicafunginAzoles Ketoconazole TopicalInhibits a cytochrome P450 enzyme 14-a-demethylase (enzyme in the ergosterol biosynthesis pathway)Mutation or overexpression of 14-a-demethylase Clotrimazole, bifonazole, econazole, miconazole, oxiconazole, sulconazole, terconazole, tioconazoleoral TopicalOverexpression of drug efflux pump(s)FluconaozleOral, IVItraconazoleOral, IVPosaconazoleOralVoriconazoleOral, IVPolyenes AmphotericinIVMain mechanism binds to ergosterol creating channels resulting in metabolite leakageAlteration or decrease in ergosterol contentNatamycinTopical (eye)NystatinTopical, oralOxidative pathways may enhance activityGriseofulvinOralActs on microtubules and interferes mitosis/cell divisionPutative mechanism drug efflux
Epidermophyton floccosum
Microsporum canis
Trichophyton rubrum
Trichophyton tonsurans
Mycology