An Unusual Cause of Cast Nephropathy in a Patient Presenting With Hypercalcemia and Renal Failure: A Case Report.

Sarcoidosis is a multisystem disease that is typically characterized by end-organ damage due to the formation of non-necrotizing granulomas in affected tissues. The lungs and lymph nodes are the most commonly affected organs; however, almost any organ can be involved. The diagnosis of sarcoidosis is based on a compatible clinical and radiological presentation, as well as histopathological evidence of non-necrotizing granulomas in affected organs. The diagnosis can be challenging, given the variability in clinical presentation and organ involvement, especially in cases without pulmonary symptoms. Renal involvement in sarcoidosis is complex and is usually attributed to nephrocalcinosis or damage to the tubulointerstitium of the kidney caused by granulomas. Cast nephropathy, which results from injury to the renal tubules due to the toxic effects of immunoglobulin light chains excreted in the urine, has previously been described in renal sarcoidosis and is most often reported in the context of hematological malignancies such as myeloma or lymphoma. We report the case of a patient who presented with renal dysfunction, hypercalcemia, and albumin-globulin (A/G) reversal mimicking multiple myeloma, but further investigations, including imaging and tissue biopsy, ultimately established a diagnosis of sarcoidosis. This report highlights the importance of a comprehensive diagnostic approach in such multisystem disorders.