Which of the following is a primary defect in Paget's disease?
Paget's disease (Osteitis deformans)

Paget's disease is characterized by increased bone turnover and enlargement and thickening of the bone, but the internal architecture is abnormal and the bone is usually brittle.
Primary defect is in osteoclasts with increased osteoclastic activity.
This results secondarily increase in osteoblastic activity (normal osteoclasts and osteoblasts act in a co-ordinated manner).
So, characteristic cellular change is a marked increase in osteoclastic and osteoblastic activity.
Bone turnover is accelerated, plasma alkaline phosphatase is raised (a sign of osteoblastic activity) and there is increased excretion of hydroxyproline in urine (due to osteoclastic activity).
Osteoblast
Osteoclast
Osteocyte
Fibroblast