Blastomycosis is a fungal infection caused by the fungus Blastomyces dermatitidis, which usually affects the lungs and skin[1]. The article describes the case of a 41-year-old man with no previous health problems who developed skin nodules, cough, pulmonary nodules and persistent arthritis of the left knee[2]. Arthritis was initially misdiagnosed as pseudogout, indicating the diagnostic difficulty of this infection[2]. The diagnosis was confirmed by CT scan, skin biopsy and fungal culture, which identified Blastomyces dermatitidis[2]. The case emphasizes that musculoskeletal manifestations of blastomycosis, especially monoarthritis, are rare and diagnostically difficult[1]. The article points out the importance of expanding the differential diagnosis according to clinical signs and the usefulness of multidisciplinary collaboration, including dermatology and mycology[2]. Skin biopsies and fungal cultures have proven to be key diagnostic methods in this case[2].