Which disease is defined by malignant plasma cells in the bone marrow?

• A. Leukemia
• B. Myeloma
• C. Lymphoma
• D. Hypersplenism

Answer: (B)

The key idea is a cancer that arises from plasma cells living in the bone marrow. In multiple myeloma, a single clone of plasma cells becomes malignant and expands within the marrow, producing a uniform (monoclonal) immunoglobulin and crowding out normal blood cell production. This malignant plasma-cell proliferation in the bone marrow is what defines the disease. It explains why patients often have bone pain from lytic lesions, anemia from suppressed normal marrow function, kidney problems from abnormal proteins, and increased susceptibility to infections. This picture is distinct from leukemia (a broader proliferation of malignant white cells in blood and marrow), lymphoma (malignancy of lymphocytes usually in lymph nodes), and hypersplenism (overactive spleen causing sequestration and destruction of blood cells). So the hallmark finding—malignant plasma cells filling the bone marrow—points to multiple myeloma.