A Diagnostic Tightrope: Carbimazole-Induced Microscopic Polyangiitis With Co-Existent Antiphospholipid Syndrome and Diffuse Alveolar Hemorrhage.

Microscopic polyangiitis (MPA) and drug-induced vasculitis represent significant diagnostic challenges. Carbimazole-induced antineutrophil cytoplasmic antibodies (ANCA)-associated vasculitis is a rare, life-threatening complication of thionamide therapy. We describe a 29-year-old female patient on long-term carbimazole therapy for 15 years who presented with acute type-1 respiratory failure due to diffuse alveolar hemorrhage. Investigations confirmed myeloperoxidase (MPO)-ANCA-positive microscopic polyangiitis with co-existent definitive antiphospholipid syndrome and atypical "immune-complex-mediated" renal deposits. This case highlights the "triple-hit" of drug-induced vasculitis, thrombosis, and immune-complex deposition. The patient's rapid clinical stabilization following the initiation of plasmapheresis emphasizes the role of this intervention as a life-saving therapeutic bridge in complex vasculitis presentations.