A 58 years old woman, who had backache and recurrent chest infections for 6 months, develops sudden weakness of the legs and urinary retention. Her investigations show hemoglobin of 7.3 gm/d1, serum calcium 12.6 mg/d1, phosphate 2.5 mg/dl, alkaline phosphatse 100 uil, serum albumin 3 gm/d1, globulin 7.1 gm/d1, and urea 178 mg/dl. What is the most likely diagnosis ?