A 45-year-old woman presents with the complaint that her toenails are thick and yellow. She is otherwise healthy and takes no medications. On examination, two toenails on the right foot and the great toenail on her left foot are affected. There is no periungual erythema, and her peripheral pulses are good. What is the best advice for this patient?

This woman has onychomycosis, which often affects the toenails in an asymmetric pattern. Onychomycosis does not usually resolve spontaneously and is more difficult to eradicate than is tinea pedis (athlete's foot). The etiologic agents include several species of yeast, mold, and dermatophytes, therefore, direct microscopy and/or fungal culture may be necessary for definitive therapy. The condition is often asymptomatic and may not require treatment. Topical therapies are effective but require daily application for many months. They have the advantage of safety. Oral treatment with terbinafine or itraconazole is more effective than topical treatment but must also be continued up to 6 months; oral antifungals carry the risk of hepatotoxicity. Yellow nail syndrome should be considered in the differential for widespread yellow nail changes and is associated with pulmonary disease and cancers. Yellow nail syndrome affects all 20 nails; a workup for systemic disease is unnecessary in the usual patient with onychomycosis.