A patient presents with bone pain, weight loss, and laboratory evidence including plasma cells in the bone marrow, elevated calcium, and proteinuria. Which disorder is most likely?

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These findings point to a plasma cell cancer that grows in the bone marrow and causes widespread bone damage. The combination of bone pain and weight loss fits a condition that directly weakens bones, producing lytic lesions and discomfort. Seeing plasma cells in the bone marrow confirms a clonal plasma cell population rather than a normal mix of blood cells. Elevated calcium comes from increased bone resorption driven by these malignant cells, a hallmark of this disease. Proteinuria reflects excess light chains—the monoclonal immunoglobulin produced by the abnormal plasma cells—passing through the kidneys. Taken together, these features form a classic pattern for multiple myeloma, a disorder characterized by bone damage, kidney involvement, and monoclonal protein production.

Osteoarthritis would not cause widespread plasma cells, high calcium, or proteinuria. Amyloidosis can involve the kidneys and cause proteinuria but does not typically involve clonal plasma cells in the bone marrow with hypercalcemia. Acute lymphoblastic leukemia involves immature lymphoblasts rather than malignant plasma cells and doesn’t usually present with the specific bone-destructive and light-chain features described.