Classification by fate: Lysine is degraded to acetoacetyl-CoA. Based on this endpoint, how is lysine classified?

Introduction / Context:
Amino acid classification hinges on carbon fate after nitrogen removal. If the carbon skeleton yields acetyl-CoA or acetoacetyl-CoA, the amino acid is ketogenic because these products cannot provide net carbons for glucose synthesis in humans. Lysine is a classic example, along with leucine.

Given Data / Assumptions:

• Lysine catabolism proceeds to acetoacetyl-CoA.
• Acetoacetyl-CoA contributes to ketone body formation and lipid synthesis.
• No net gluconeogenesis occurs from acetyl-derived carbons in animals.

Concept / Approach:
Match the endpoint to the classification scheme. Because acetoacetyl-CoA is a ketogenic product, lysine is labeled ketogenic-only. This differs from dual-fate amino acids (e.g., isoleucine, phenylalanine) that produce both TCA intermediates (glucogenic) and acetyl-derived products (ketogenic).

Step-by-Step Solution:

Verification / Alternative check:
Standard pathway charts and clinical nutrition references classify lysine and leucine as the two purely ketogenic amino acids, corroborating this answer.

Why Other Options Are Wrong:

• Glucogenic: would require formation of pyruvate or TCA intermediates.
• Both equally: lysine is not dual-fate.
• None of these / invented terms: do not reflect accepted biochemical categories.

Common Pitfalls:
Assuming essential amino acids are all glucogenic; essentiality does not determine glucogenic or ketogenic status.

Final Answer:
Ketogenic amino acid