Which pair of statements best describes an essential amino acid in human nutrition?

• A. It can be synthesized by the human body from any carbon skeleton and therefore does not need to be obtained from the diet.
• B. It is not required for protein synthesis but is needed solely for urea cycle function.
• C. It is synthesized only in the liver and is required primarily as an energy source during fasting.
• D. It cannot be synthesized in adequate amounts by the human body and therefore must be obtained from the diet.

Answer: (D)