Amino acid metabolism (classification): Which option below is the best-described purely ketogenic amino acid in humans?

Introduction / Context:
Amino acids are termed ketogenic if their carbon skeletons are degraded exclusively to acetyl-CoA or acetoacetyl-CoA, yielding ketone bodies or entering fatty acid synthesis, but not producing net glucose. Only a few are purely ketogenic. Identifying them is a common test question in medical and nutrition biochemistry.

Given Data / Assumptions:

• Lysine and leucine are purely ketogenic.
• Valine is purely glucogenic.
• Tryptophan and phenylalanine are both glucogenic and ketogenic.

Concept / Approach:
Classify the listed amino acids by catabolic endpoints. The hallmark of a purely ketogenic amino acid is exclusive conversion to acetyl-CoA/acetoacetyl-CoA without generating gluconeogenic intermediates. Lysine fits this definition. Tryptophan and phenylalanine generate mixed products, while valine is routed to succinyl-CoA and is glucogenic.

Step-by-Step Solution:

Verification / Alternative check:
Metabolic pathway charts consistently annotate lysine as producing acetoacetyl-CoA, confirming its purely ketogenic status.

Why Other Options Are Wrong:

• Tryptophan: yields both glucogenic and ketogenic products.
• Valine: glucogenic via propionyl-CoA to succinyl-CoA.
• Phenylalanine: both glucogenic and ketogenic (via fumarate and acetoacetate).
• None of these: incorrect because lysine is correct.

Common Pitfalls:
Memorization errors mixing leucine/lysine with other essential amino acids; always anchor the rule that leucine and lysine are the two purely ketogenic amino acids.

Final Answer:
Lysine