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

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

Answer: (C)

Malignant plasma cells in the bone marrow point to a plasma cell neoplasm, most characteristically multiple myeloma. In this disease, clonal plasma cells accumulate in the marrow, crowding out normal blood-forming cells and often secreting a monoclonal immunoglobulin (the M protein) detectable in blood or urine. This marrow invasion leads to bone destruction with lytic lesions, bone pain, anemia from reduced red cell production, hypercalcemia, and kidney dysfunction due to light chains. The other conditions involve different cell types or sites—leukemia involves malignant leukocytes in blood/marrow but not specifically plasma cells; lymphoma centers on lymphoid tissues; hypersplenism is about sequestration in an enlarged spleen. Thus, the description best fits multiple myeloma.