A 58 year old woman, who had backache and recurrent chest Infections for 6 months, develops sudden weakness of the legs and urinary retention. Her investigations show a hemoglobin of 7.3 gm/ dl, serum calcium 12.6 mg/dl, phosphate-2.5 mg/ dl, alkaline phosphatase-1OOu/l, serum albumln- 3 gm/di, globulin - 7.1 gm/dl and urea - 178 mg/d. What is the most likely diagnosis -