[Differential diagnosis of onychomycosis].

Almost 50% of nail pathology is caused by onychomycosis. However, the nail has a limited reservoir of clinical expressions and many conditions may mimic onychomycosis. Its differential diagnosis should be known in order to avoid useless or even toxic treatments. When confronted with ungual alterations restricted to one or some nails, it is fair to evoke onychomycosis but one should keep in mind repeated traumata to the toenails, psoriasis and chronic paronychia on the fingernails. Involvement of a large number of nails, especially all twenty nails, should not first suggest a fungal infection but rather an inflammatory or systemic condition. It is mandatory to always sample the nail for mycological examination before prescribing any systemic treatment.