What percentage of the body's phosphate is stored in bone?

• A. 85%
• B. 75%
• C. 95%
• D. 65%

Answer: (A)

Phosphate is stored predominantly in bone, where it is part of the hydroxyapatite mineral that gives bone its strength. This makes bone the body's major reservoir for phosphate. In adults, about 85% of total body phosphate is in bone, with roughly 14–15% in soft tissues and around 1% in extracellular fluid. Because most phosphate is in bone, serum phosphate is buffered by exchange with this reservoir, while soft tissues hold only a small pool. The other percentage options would imply a distribution that doesn’t match this well-established pattern. Hormones like PTH, vitamin D, and FGF23 help regulate how much phosphate is released from bone and how much is excreted by the kidneys to maintain balance.