In Multiple Myeloma, what is the characteristic cell found in EXCESS in the bone marrow?

Multiple Myeloma is a malignant proliferation of plasma cells (terminally differentiated B lymphocytes) in the bone marrow. These malignant plasma cells produce a monoclonal immunoglobulin (M-protein/paraprotein) detectable in serum (SPEP) or urine (Bence Jones protein). Diagnosis requires >10% plasma cells on bone marrow biopsy. Oral manifestations include jaw pain, osteolytic lesions ('punched-out' radiolucencies), pathological fractures, and amyloid deposits.