## Core Concept
The question tests the understanding of amino acid metabolism, specifically the categorization of amino acids based on their ability to be converted into glucose (glucogenic) or ketone bodies (ketogenic), or both. Amino acids can be classified into glucogenic, ketogenic, or both, depending on their metabolic end products.

## Why the Correct Answer is Right
Glucogenic amino acids can be converted into glucose through gluconeogenesis. They are typically metabolized into intermediates that can enter the gluconeogenic pathway, such as pyruvate, oxaloacetate, phosphoenolpyruvate, and others. Among the given options, **Leucine** is purely ketogenic, meaning it can only be converted into acetyl-CoA and acetoacetate (ketone bodies) and not into glucose. This is because leucine's carbon skeleton enters the citric acid cycle at the level of acetoacetyl-CoA or acetyl-CoA, which cannot be used for gluconeogenesis.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is not provided, but based on standard glucogenic amino acids, examples include Alanine, Aspartate, and Glutamate, which can all be converted into glucose.
- **Option B:** Similarly, this option is not provided, but assuming it represents another amino acid, if it's glucogenic, it can be converted into glucose.
- **Option C:** Again, not provided, but if it's a glucogenic amino acid, it supports gluconeogenesis.
- **Option D:** This is the correct answer based on the question; if it's not purely glucogenic, it implies it could be ketogenic or both.

## Clinical Pearl / High-Yield Fact
A key point to remember is that **Leucine** is an example of a purely ketogenic amino acid. This classification is crucial in understanding the metabolic fate of amino acids, especially in conditions like diabetes or fasting states where gluconeogenesis and ketogenesis are significant.

## Correct Answer: D. Leucine