Light chain deposition disease masquerading as hypophosphatemic osteomalacia: a case report.

Light chain deposition disease (LCDD) is characterized by the deposition of monotypic immunoglobulin light chains in multiple organs. The kidneys are usually affected in cases of LCCD, and they typically present with mesangial nodule formation, extensive monoclonal light chain deposition, proteinuria, and renal insufficiency. We report a rare case of LCDD that presented with bone pain and proximal amyosthenia secondary to renal tubular phosphate loss. A 42-year-old woman presented with progressive pain in the waist and bilateral hips, along with increasing amyosthenia in both lower limbs that had persisted for more than a year. Upon investigation, she was diagnosed with hypophosphatemic osteomalacia (HO). She also had aminoaciduria, renal glucosuria, hypokalemia, hypouricemia, and renal tubular proteinuria with elevated serum kappa light chains. A renal biopsy revealed the presence of fibrillar deposits of kappa light chains along the glomerular basement membranes, as observed using electron microscopy and immunofluorescence. Congo red staining was negative. Based on the ‌finding of abnormal plasma cells comprising 1.1%‌ of cellular elements in a bone marrow biopsy, we made a diagnosis of monoclonal gammopathy of renal significance. The patient received chemotherapy and exhibited positive clinical and biochemical responses. This case describes a unique presentation of renal injury due to LCDD masquerading as HO. The aim of this report is to highlight that HO in adults is usually acquired and that addressing the underlying etiology can result in a successful cure.