Atypical Vasculitis Phenotype with Concomitant Medium- and Small-Vessel Involvement: A Case Report Highlighting Diagnostic and Pathological Challenges.

Systemic vasculitis is traditionally classified according to the predominant size of affected vessels; however, phenotypes affecting multiple vascular calibers may not be adequately captured by rigid definitions and require integrated clinicopathological interpretation. Here we report the case of a 70-year-old Caucasian man who presented with persistent systemic inflammation initially suggestive of infection. Although inflammatory markers decreased during antibiotic therapy, residual inflammation persisted, and the subsequent clinical course was characterized by myalgia, pulmonary involvement, and lymphadenopathy. Muscle biopsy revealed noninflammatory myopathic changes, consistent with normal creatine kinase levels. Following the initiation of corticosteroid therapy, the patient rapidly developed purpura and acute nephritic syndrome. Kidney biopsy demonstrated IgA glomerulonephritis (Oxford scores: M0, E0, S0, T0, and C1), interpreted as a manifestation of IgA vasculitis (IgAV), along with sclerosing phase of necrotizing arteritis involving arcuate arteries. These findings demonstrated the coexistence of IgA-mediated glomerular/small-vessel manifestations and chronic medium-vessel arterial pathology but did not establish the exact pathological process leading to the clinical manifestation. Treatment with corticosteroids and cyclophosphamide resulted in rapid clinical remission. This case illustrates a diagnostically challenging vasculitis phenotype involving different vascular compartments that cannot be assigned to a single conventional category. The case highlights the importance of integrated clinicopathological correlation and detailed histopathological assessment in resolving diagnostic uncertainty and guiding immunosuppressive therapy. Recognition of vasculitis as a dynamic disease spectrum rather than a rigidly compartmentalized entity is essential for timely diagnosis and optimal management.