How is cysteine added to parenteral nutrition, given its instability in aqueous solutions?

• A. It is added immediately prior to administration
• B. It is mixed with lipids
• C. It is included in the base solution
• D. It is added after storage at room temperature

Answer: (A)

Cysteine is unstable in aqueous parenteral nutrition solutions, tending to degrade and oxidize if kept in solution for any length of time. To preserve its intended dose and prevent instability-related issues, it is added as a sterile, ready-to-use substance immediately before administration. This timing avoids degradation that would occur if it were included in the base solution or stored, and keeps it separate from lipid emulsions, which have their own handling requirements.