Why is cysteine not included in usual amino acid solutions for PN?

• A. It is unstable in solution
• B. It reacts with other amino acids
• C. It is non-essential
• D. Because of solubility concerns

Answer: (D)

The main idea is the difficulty of keeping amino acids soluble in intravenous nutrition solutions. Cysteine has a sulfhydryl group that makes it poorly soluble in the aqueous, near-neutral pH solutions used for standard PN. It also tends to oxidize and can precipitate, which can cause issues with IV compatibility and safety. Because IV nutrition must be fully soluble to be safely infused, cysteine is not included in the usual amino acid mix unless specifically needed, and it may be added separately or with adjustments when clinically required.

Other options don’t fit as the primary reason: cysteine isn’t avoided because it’s non-essential in all contexts, nor primarily because it reacts with other amino acids; the core constraint here is solubility and stability in the PN solution.