What is burosumab and for which condition is it used?

• A. An oral phosphate binder used to reduce phosphate in CKD
• B. A calcimimetic agent used for secondary hyperparathyroidism
• C. A monoclonal antibody against FGF23 used for treating X-linked hypophosphatemia (XLH)
• D. A vitamin D analog used for the treatment of nutritional rickets

Answer: (C)

Burosumab is a monoclonal antibody that targets FGF23, addressing the underlying problem in X-linked hypophosphatemia (XLH). In XLH, mutations lead to excess FGF23 activity, which causes the kidneys to waste phosphate and suppresses production of active vitamin D. This sequence lowers serum phosphate and impairs bone mineralization, leading to rickets in children and osteomalacia in adults. By neutralizing FGF23, burosumab increases phosphate reabsorption in the kidneys and raises 1,25-dihydroxyvitamin D levels, improving phosphate availability and promoting better bone mineralization.

The other options describe treatments for different conditions: an oral phosphate binder is used to reduce phosphate in chronic kidney disease, a calcimimetic modulates the calcium-sensing receptor for secondary hyperparathyroidism, and a vitamin D analog is used for nutritional rickets without specifically addressing FGF23-driven phosphate wasting in XLH.