**Core Concept**
The question tests the understanding of amino acid metabolism, specifically the distinction between glucogenic and ketogenic amino acids. Amino acids can be purely glucogenic, purely ketogenic, or both, depending on their metabolic pathways.

**Why the Correct Answer is Right**
Leucine is an example of an amino acid that is purely ketogenic, meaning it can be converted into acetyl-CoA or acetoacetyl-CoA, which are then used to produce ketone bodies but cannot be used to synthesize glucose. This is because leucine's carbon skeleton is metabolized into acetyl-CoA and then into ketone bodies without any possibility of being converted back into glucose.

**Why Each Wrong Option is Incorrect**
**Option A:** This choice is incorrect because it does not represent leucine.
**Option B:** This choice is incorrect because it does not represent leucine.
**Option D:** This choice is incorrect because it does not represent leucine.

**Clinical Pearl / High-Yield Fact**
It's crucial to remember that leucine is a purely ketogenic amino acid, which has implications for understanding metabolic states, particularly in diabetes or fasting states where the body relies more heavily on ketone bodies for energy.

**Correct Answer:** D. Leucine