Light chain (AL) amyloidosis is the result of a clonal plasma cell disorder which causes organ damage by deposition of misfolded light chains. Kidney is a common site of amyloid deposition. Proteinuria, usually in nephrotic range and unexplained renal insufficiency are the main manifestations of renal injury. We report a unique case of renal involvement by AL amyloidosis masquerading as metabolic bone disease. 38 year old male patient presented with progressively increasing diffuse bony pains, low backache and proximal weakness of both lower limbs since two years. On investigation, he was detected to have hypophosphatemic osteomalacia due to renal phosphate loss which was fibroblast growth factor 23 (FGF23)- independent. He also had nephrotic range low molecular weight proteinuria. Renal biopsy to ascertain the aetiology revealed deposition of amyloid fibrils in the glomerular mesangium on electron microscopy. Its characterization by immunofluorescence (IF) was consistent with immunoglobulin light chain (AL) amyloidosis. In the absence of a demonstrable plasma cell clone on bone marrow biopsy, we made a diagnosis of monoclonal gammopathy of renal significance (MGRS). He was treated with chemotherapy following which there was symptomatic improvement and reduction in phosphaturia. This case describes a unique presentation of renal injury due to AL amyloidosis masquerading as hypophosphatemic osteomalacia. The aim of this report is to highlight that hypophosphatemia in adults is usually acquired and treatment of underlying etiology results in cure, unlike in children where genetic counseling and phosphate replacement is the mainstay of treatment.

轻链（AL）淀粉样变性病是克隆性浆细胞疾病的结果，该疾病会因错误折叠的轻链沉积而导致器官损伤。肾脏是淀粉样蛋白沉积的常见部位。蛋白尿（通常在肾病范围内）和无法解释的肾功能不全是肾损伤的主要表现。我们报道了AL淀粉样变性病伪装成代谢性骨病的肾脏受累的独特案例。38岁的男性患者自两年以来表现出逐渐增加的弥漫性骨痛，低腰酸和下肢近端无力。经调查，由于肾磷酸盐丢失，他被发现患有低磷酸盐血症性骨质软化症，而肾磷酸盐是成纤维细胞生长因子23（FGF23）独立的。他还患有肾病范围的低分子量蛋白尿。肾活检以确定病因，在电子显微镜下发现淀粉样纤维沉积在肾小球系膜中。通过免疫荧光（IF）表征其与免疫球蛋白轻链（AL）淀粉样变性一致。在骨髓活检中没有可证实的浆细胞克隆的情况下，我们诊断出具有肾脏意义的单克隆丙种球蛋白病（MGRS）。他接受了化学疗法的治疗，之后出现了症状改善和血尿减少。该病例描述了由于AL淀粉样变性病伪装为低磷酸盐血症性骨软化症而引起的肾损伤的独特表现。本报告的目的是强调通常会获得成年人的低磷血症，并且通过对潜在病因的治疗可以治愈，