The article discusses modern aspects of the formation of resistance of mycotic infections to antifungal drugs, which is one of the reasons for the loss of the expected clinical efficacy of therapy. Factors such as the appointment of antimycotics without laboratory tests necessary to confirm or exclude the fungal nature of nail damage, misinterpretation of diagnostic test results, irrational treatment tactics, and self-treatment contribute to a decrease in the sensitivity of pathogens of superficial skin mycosis and onychomycosis. Changes in the nail apparatus are characteristic not only for fungal invasion, but also for onychodystrophies and a number of other nosologies. The similarity of clinical manifestations of onychopathy of various etiologies creates difficulties in differential diagnosis and can lead to therapeutic errors. The article presents the main clinical characteristics of nail pathology of various origins: onychomycosis (a chronic fungal infection of the nails that requires etiotropic therapy with antifungal agents), and onychodystrophy (different non-infectious changes in the nail plate, in which the use of antimycotics is not indicated). Modern methods of laboratory and instrumental diagnosis of onychomycosis are described, aspects of differential diagnosis of onychomycosis with all types of non-infectious dystrophies of the nail plates, including nail lesions in diseases such as psoriasis, lichen planus, eczema, are discussed.

Keywords: drug sensitivity, dermatophytes, onychodystrophy, laboratory diagnostics, onychomycosis, resistance