Muscle-predominant vasculitic myopathy heralding renal MPO-ANCA-associated vasculitis: A case-based mini-review of diagnostic traps and steroid-sparing induction.

ANCA-associated vasculitis is a systemic autoimmune disease that typically affects the kidneys, lungs, and upper respiratory tract. Muscle-predominant ANCA-associated vasculitis is exceedingly rare and frequently misdiagnosed as idiopathic inflammatory myopathy due to overlapping clinical and imaging findings. We report a 73-year-old male presenting with progressive proximal muscle weakness, normal creatine kinase, and MRI findings suggestive of myositis. Acute kidney injury prompted further investigation, and muscle and renal biopsies confirmed ANCA-associated vasculitis. The patient improved with rituximab and avacopan, an emerging glucocorticoid-sparing therapy. This case-based mini-review summarizes the epidemiology, diagnostic challenges, biopsy timing, and treatment strategies for muscle-predominant ANCAassociated vasculitis, underscoring the importance of tissue diagnosis and early, targeted therapy in atypical presentations.