A 56-year-old male presents with lower back pain for over 6 months, accompanied by irregular fever, urinary frequency, and dysuria. Laboratory tests show: urinary protein (+++), white blood cell (WBC) count 10-15/HPF, positive Bence Jones protein in urine. Serum protein electrophoresis (SPEP) reveals a monoclonal protein (M protein) of 0.395 (39.5%) between β and γ regions. A provisional diagnosis of multiple myeloma (MM) is made. Which of the following laboratory findings is most definitive for confirming the diagnosis?