Calcitonin inhibits osteoclast activity, reducing bone resorption and lowering blood calcium. Which statement correctly describes its action on bone?

• A. Stimulates osteoclast activity
• B. Inhibits osteoclast activity
• C. Increases intestinal absorption of calcium
• D. Enhances osteoblast bone formation

Answer: (B)

Calcitonin acts to lower blood calcium by slowing bone resorption. It does this by inhibiting osteoclasts, the cells that break down bone and release calcium into the bloodstream. With osteoclast activity reduced, less calcium is released from bone, so blood calcium levels fall. It doesn't mainly promote calcium absorption in the gut, nor does it directly boost osteoblast activity to form new bone; its primary effect is to prevent bone breakdown. So the best description of its action on bone is that it inhibits osteoclast activity.