How does parathyroid hormone (PTH) regulate bone resorption?

• A. PTH directly stimulates osteoclasts to resorb bone
• B. PTH inhibits osteoblasts
• C. PTH stimulates osteoblasts to express RANKL, which activates osteoclasts
• D. PTH blocks calcium release from bone

Answer: (C)

PTH raises blood calcium by triggering bone resorption through an indirect signaling pathway involving osteoblasts. When PTH binds to receptors on osteoblasts and other stromal cells, it increases the expression of RANKL (receptor activator of nuclear factor kappa-Β ligand) and decreases the production of osteoprotegerin (OPG), a decoy receptor that normally limits RANKL. RANKL then binds to RANK on osteoclast precursors, promoting their maturation and activating mature osteoclasts to resorb bone. This chain releases calcium from bone into the bloodstream. So, the hormone’s effect on bone resorption is mediated by osteoblasts via the RANKL–OPG system, not by directly stimulating osteoclasts or inhibiting osteoblasts, and it does not block calcium release from bone.