A woman presented to her primary care physician with right shoulder pain. There was no history of trauma to the shoulder. Her past medical history was notable for a left nephrectomy 25 years earlier, secondary to complications of sepsis. At time of presentation, the woman had hypertension, osteoarthritis, and mild renal insufficiency with proteinuria. Radiographs of her shoulder revealed a large bony lesion of the proximal humerus with an associated pathologic fracture. Other lytic lesions were identified in the glenoid, the distal right clavicle, and the inferior scapula. Laboratory values included calcium, 9.9 mg/dL (2.48 mmol/L) (reference range, 8.4– 10.2 mg/dL [2.05–2.55 mmol/L]); phosphorus, 3.8 mg/dL (1.23 mmol/L) (2.7–4.5 mg/dL [0.74–1.52 mmol/L]); glucose, 84 mg/dL (4.7 mmol/L) (70–105 mg/dL [2.8–4.4 mmol/L]); sodium, 140 mEq/L (133–145 mEq/L); potassium, 3.6 mEq/L (3.3–5.1 mEq/L); creatinine, 1.4 mg/dL (107 mmol/L) (0.4–1.2 mg/dL [8–31 mmol/L]); serum urea nitrogen, 16 mg/dL (5.7 mmol/L) (6–19 mg/dL [2.9–8.2 mmol/L]); and albumin, 3.9 g/dL (3.9–4.8 g/dL). A urine and serum immunofixation electrophoresis revealed a monoclonal gammopathy of free k light chain. A bone marrow aspirate and biopsy was done to rule out multiple myeloma. The bone marrow aspirate was also sent for flow cytometry. The bone marrow biopsy showed hypercellular marrow (60%–70%) with replacement by plasma cells. Several megakaryocytes were noted, and erythroid and myeloid precursors were decreased. The plasma cells were atypical and had large nuclei. The bone marrow aspirate stained with Wright-Giemsa showed several crystalline structures in the cytoplasm of the plasma cells (50%–60%) (Figure, oil immersion, original magnification 3760). They were unstained, needlelike in shape, and were not birefringent. Cytoplasmic immunoglobulin stains revealed k light chain restriction; however, the crystals remained unstained. The bony trabeculae were unremarkable. Prussian blue stain showed iron present mostly in macrophages. No ringed sideroblasts were noted. Flow cytometry confirmed a clon-