Which amino acid is conditionally indispensable because it can be synthesized from methionine and serine?

• A. Tyrosine
• B. Cysteine
• C. Proline
• D. Arginine

Answer: (B)

Cysteine is conditionally indispensable because it can be made in the body from methionine and serine through the transsulfuration pathway. In this route, methionine donates sulfur and is converted to homocysteine, which, with serine, forms cystathionine and then cysteine. When this pathway functions normally and methionine intake is adequate, cysteine is nonessential. However, if methionine is scarce, the pathway is impaired, or in infants whose ability to synthesize cysteine is immature, dietary cysteine becomes necessary. The other amino acids listed are derived from different precursors or pathways (tyrosine from phenylalanine, proline from glutamate, arginine with special growth-related requirements), so cysteine specifically hinges on methionine and serine for its synthesis.