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-100u/I, serum albumin-3 gm/dl, globulin - 7.1 gin/dl and urea - 178 mg/d.What is the most likely diagnosis -