**Core Concept**
Denosumab is a human monoclonal antibody that targets RANKL (Receptor Activator of NF-κB Ligand), a protein essential for the formation, function, and survival of osteoclasts. By inhibiting RANKL, denosumab reduces osteoclast activity, leading to decreased bone resorption and increased bone density.

**Why the Correct Answer is Right**
Denosumab is used in the treatment of osteoporosis, particularly in postmenopausal women and men with high risk of fractures. It is also used in the treatment of bone metastases in patients with solid tumors, such as breast, lung, and prostate cancer. By inhibiting osteoclast activity, denosumab decreases the risk of fractures and bone-related complications in these patients.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because denosumab is not primarily used to treat rheumatoid arthritis, although it may be used off-label in some cases to treat bone erosions associated with this condition.

**Option B:** This option is incorrect because while denosumab can be used to treat bone metastases, it is not primarily used to treat prostate cancer itself.

**Option C:** This option is incorrect because denosumab is not used to treat osteoarthritis, which is a degenerative joint disease characterized by cartilage breakdown and inflammation.

**Clinical Pearl / High-Yield Fact**
Denosumab is a valuable treatment option for patients with osteoporosis and bone metastases, but it is not without risks, including increased risk of hypocalcemia and skin infections.

**Correct Answer:** C. Bone metastases in patients with solid tumors.