Which of the following amino acids are exclusively ketogenic?

The correct option is Leucine.

Amino acids can be classified based on the metabolic pathways of their carbon skeletons into three main categories:
1. Glucogenic amino acids: These are degraded into intermediates of gluconeogenesis (such as pyruvate, α-ketoglutarate, succinyl-CoA, fumarate, or oxaloacetate) and can be used to synthesize glucose.
2. Ketogenic amino acids: These are degraded directly into acetyl-CoA or acetoacetyl-CoA, which are precursors for ketone bodies and fatty acids, but cannot be used for net glucose synthesis.
3. Both glucogenic and ketogenic amino acids: These can yield precursors for both pathways.

Among the twenty standard amino acids, only two are exclusively ketogenic:
- Leucine
- Lysine

Let's evaluate the options provided:
- Asparagine: Exclusively glucogenic. It is converted to aspartate and then to the citric acid cycle intermediate oxaloacetate.
- Proline: Exclusively glucogenic. It is metabolized to glutamate, which is then converted to α-ketoglutarate.
- Threonine: Both glucogenic and ketogenic. Its breakdown can yield both succinyl-CoA (glucogenic) and acetyl-CoA (ketogenic).
- Leucine: Exclusively ketogenic. Its catabolism yields acetyl-CoA and acetoacetate, making it unable to contribute to net glucose synthesis.

Therefore, Leucine is the correct exclusively ketogenic amino acid.