Which is the biomarker of osteoclast activity (bone resorption)?

• A. Serum alkaline phosphatase
• B. Serum 25-hydroxyvitamin D
• C. Urine pyridinoline cross-links
• D. Serum osteocalcin

Answer: (C)

The concept being tested is how to distinguish bone resorption markers from bone formation markers. Osteoclasts drive bone resorption and release collagen breakdown products into the circulation, including pyridinoline cross-links. These cross-links, such as pyridinoline and deoxypyridinoline, are excreted in urine and serve as a specific measure of bone resorption activity. That makes urinary pyridinoline cross-links the best indicator of osteoclast-driven bone turnover.

In contrast, serum alkaline phosphatase and serum osteocalcin are tied to osteoblast activity and bone formation, reflecting new bone creation rather than resorption. Serum 25-hydroxyvitamin D reflects vitamin D status, which influences bone health but is not a direct marker of osteoclast-mediated resorption.