A 62 year old man has bone pain at his hips and back. On further investigatioin, alkaline phosphatase was found to be elevated in his blood. An X-ray shows multifocal sclerotic patches in the skull. The diagnosis is : 1 Paget's disease 2 Osteoporosis 3 Osteomalacia 4 Multiple Myeloma 5 Ankylosing Spondylitis