Why is cysteine considered likely essential in preterm infants but not in adults?

• A. Because preterms cannot synthesize methionine from cysteine
• B. Because the liver trans-sulfuration enzyme is underdeveloped in preterms, preventing cysteine synthesis
• C. Because adults can't absorb cysteine from the gut
• D. Because preterms produce too much cysteine

Answer: (B)

Cysteine becomes conditionally essential in preterm infants because the liver’s trans-sulfuration pathway, which converts methionine to cysteine, is immature. This pathway relies on enzymes such as cystathionine beta-synthase and downstream steps to transform homocysteine into cysteine. In preterms, these enzymes are underdeveloped, so the body can’t effectively synthesize cysteine from methionine. Adults have fully mature trans-sulfuration activity, allowing endogenous cysteine production to meet needs, so cysteine is not essential for them. The other ideas don’t fit because absorption of cysteine in adults is not the limiting factor, there isn’t excess cysteine production in preterms, and the key issue is the inability to synthesize cysteine endogenously due to immature liver enzymes.