A 58 years old woman, who had backache and recurrent chest infections for 6 months, develops sudden weakness of the legs and urinary retention. His investigations show a hemoglobin of 7.3 gm/dl, serum calcium 12.6 mg/dl, phosphate-2.5 mg/dl, alkaline phosphatase-100 u/1, serum albumin-3 gm/dl, globulin-7.1 gm/dl and urea-178 mg/d. What is the most likely diagnosis?