Which hormone inhibits osteoclast activity and reduces bone resorption?

• A. Parathyroid hormone
• B. Vitamin D
• C. Estrogen
• D. Calcitonin

Answer: (D)

The main idea here is how hormones regulate bone remodeling by controlling osteoclast activity. Calcitonin directly inhibits osteoclasts, the cells responsible for bone resorption, so it reduces the breakdown of bone and lowers blood calcium when calcium is high. This direct action on osteoclast activity is what makes calcitonin the best fit for inhibiting bone resorption.

Parathyroid hormone, in contrast, raises blood calcium largely by stimulating osteoclast activity (indirectly through signals from osteoblasts), so it increases bone resorption rather than inhibiting it. Vitamin D helps with calcium absorption and bone mineralization but doesn’t primarily act by turning off osteoclasts. Estrogen does reduce bone resorption as well, partly by limiting osteoclast formation and promoting apoptosis, but the question’s focus is on direct inhibition of osteoclast activity, which calcitonin accomplishes most specifically.