Adding cysteine to parenteral nutrition improves calcium and phosphorus solubility by which mechanism?

• A. Increasing
• B. Lowering
• C. Neutralizing
• D. Raising

Answer: (B)

Lowering the pH of the parenteral nutrition solution is what cysteine does, and that acidification improves calcium and phosphate solubility. Calcium phosphate tends to precipitate more at higher pH because more phosphate is present in the highly charged, insoluble PO4^3− form. When cysteine lowers the solution’s pH, phosphate shifts toward protonated forms like H2PO4− and HPO4^2−, which don't bind calcium as readily to form insoluble calcium phosphate. So the added cysteine keeps calcium and phosphate in solution by acidifying the mixture, reducing precipitation.