Cocaine-Triggered PR3-ANCA Vasculitis Localized to a Post-Surgical Neck Field: A Case of Locus Minoris Resistentiae in Drug-Induced Autoimmunity

<b>Background and Clinical Significance:</b> Cocaine-induced vasculitis (CIV), especially when associated with PR3-ANCA positivity, can be very similar both clinically and serologically to idiopathic granulomatosis with polyangiitis (GPA). The distinction between these entities is crucial due to the different etiologies, treatment strategies, and prognoses. We present a unique case of CIV that manifested exclusively in a previously dissected neck area-an example of the locus minoris resistance phenomenon-and was initially misinterpreted as skin melanoma recurrence. <b>Case presentation:</b> A 59-year-old man with a history of skin melanoma (pT4b, left pectoral region) and a previous modified radical neck dissection presented in 2024 with new onset of painful subcutaneous nodules and ulcerative lesions at the surgical site. The imaging procedures (CT and PET-CT) raised the suspicion of locoregional malignant recurrence. However, histology revealed necrotizing granulomatous inflammation without tumor cells. Extensive infectious and autoimmune investigations ruled out alternative causes. Subsequently, the patient developed a perforation of the nasal septum and ulcers on the oral mucosa. PR3-ANCA was strongly positive (up to 49 U/mL). Urine toxicology revealed intranasal cocaine use. A diagnosis of cocaine-induced PR3-ANCA vasculitis was made. After immunosuppressive therapy (high-dose glucocorticoids and methotrexate) and substance withdrawal counseling, the patient showed significant clinical improvement. <b>Conclusions:</b> This case highlights the importance of including CIV in the differential diagnosis of granulomatous or ulcerative lesions, especially when they are localized to previous surgical sites. The presentation illustrates the concept of locus minoris resistentiae and highlights the role of toxicological testing in atypical ANCA-positive disease.