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A woman in her 20s with a mechanical mitral valve replacement on long-term warfarin therapy presented with frank haematuria and acute kidney injury (AKI). The initial clinical impression was anticoagulant-related nephropathy. However, due to the persistence of urinary abnormalities, further evaluation was undertaken. Renal biopsy revealed immunoglobulin A nephropathy with lambda light chain predominance, raising concern for an underlying monoclonal gammopathy. The patient was managed conservatively with antiproteinuric therapy alone, without immunosuppression, and demonstrated favourable renal recovery. This case highlights the diagnostic importance of renal biopsy and thorough clinical reassessment in patients with haematuria and AKI, particularly in the context of anticoagulation.