Why is cysteine not included in standard parenteral amino acid solutions? How is it provided for infants?

• A. It is not a component because it is unstable in aqueous solution.
• B. Cysteine is unstable in aqueous solution, so it is added immediately prior to administration.
• C. It is essential but excluded due to cost
• D. It is replaced by taurine in neonates

Answer: (B)

Cysteine has a sulfhydryl group that is easily oxidized in aqueous parenteral solutions, so free cysteine is unstable and can degrade or change the composition of the amino acid mix. Because of this, it isn’t included in standard ready-to-use parenteral amino acid solutions. For infants, cysteine is provided by adding it as a separate IV amino acid additive immediately before administration (often as L-cysteine hydrochloride) to ensure the required dose is given without compromising stability. It isn’t about cost, and taurine is not a substitute for cysteine in neonates.