Paget's disease of bone involves a markedly elevated serum alkaline phosphatase with normal calcium and phosphate. The cell type primarily hyperactive in the lytic phase and responsible for the elevated alkaline phosphatase is:

• A Osteoblasts alone — hyperfunctioning and secreting excess ALP
• B Osteocytes releasing RANKL and driving paracrine osteoclast activation
• C Osteoclasts — driving excessive bone resorption in the lytic phase; ALP elevation reflects subsequent compensatory osteoblastic activity ✓
• D Chondrocytes undergoing endochondral ossification

Explanation

Paget's disease begins with exaggerated osteoclast activity (driven by viral paramyxovirus antigens activating RANKL signalling), producing the lytic phase (osteoporosis circumscripta on skull X-ray, blade of grass sign in long bones). The compensatory osteoblastic response produces abundant but disorganised woven bone — explaining markedly elevated alkaline phosphatase (an osteoblast marker). Serum calcium and phosphate remain normal because bone turnover is coupled, though hypercalcaemia can occur with immobilisation or malignant transformation. Bisphosphonates (zoledronate) normalise ALP.

Reference: Maheshwari Essential Orthopaedics, 6th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.