A patient with multiple myeloma has a bone marrow biopsy showing a marked increase in which cell type?

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