Which cell type is found in increased numbers in the bone marrow of a patient with multiple myeloma?

Multiple myeloma is a malignant proliferation of plasma cells within the bone marrow, leading to monoclonal antibody (M-protein) overproduction, bone destruction (including characteristic 'punched-out' radiolucent lesions, sometimes seen in the jaws), anemia, renal impairment, and hypercalcemia.