• Clarissa A Cassol, Pawan K Rao, and Juarez R Braga.
• Department of Pathology, Renal Pathology Division, the Ohio State University Wexner Medical Center.
• Medicine (Baltimore). 2018 Dec 1; 97 (52): e13799.

RationaleAcute renal failure developing over a short period of time with evidence of glomerular disease by urine sediment microscopy characterizes the clinical syndrome of rapidly progressive glomerulonephritis (RPGN), of which the most common causes are ANCA-associated glomerulonephritis (GN), immune-complex mediated GN and anti-GBM disease.Patient ConcernsThis was a middle-aged gentleman who presented with acute renal failure and a positive PR3-ANCA.DiagnosisRenal biopsy showed an unusual combination of PR3-ANCA GN with focal crescents, monoclonal immunoglobulin deposition disease (MIDD) and mesangial IgA deposition on renal biopsy.InterventionsSerum and urine protein electrophoresis (UPEP) and immunofixation showed no detectable monoclonal paraprotein; bone marrow biopsy was negative for plasma cell neoplasia. He received high dose steroids and rituximab.OutcomesThe patient did not respond to treatment and progressed to end-stage renal failure within 2 months after presentation.LessonsTo our knowledge, the simultaneous occurrence of MIDD, PR3-ANCA and mesangial IgA has not been reported. This case highlights not only the diagnostic but also the therapeutic challenges that such a complex case presentation poses to clinicians, where the culprit may not always be what would seem most obvious (such as ANCA in a patient with RPGN) but may, in fact, be an underlying and unsuspected disease, or possibly a combination of both.

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