**Core Concept**
The question tests the understanding of amino acid metabolism, specifically those that can enter the Tricarboxylic Acid (TCA) cycle and are involved in gluconeogenesis and ketogenesis. Amino acids can be glucogenic, ketogenic, or both, depending on their metabolic fate.

**Why the Correct Answer is Right**
The correct amino acid should be one that can be converted into intermediates of the TCA cycle, which can then be used for gluconeogenesis, and also lead to the production of ketone bodies, making it ketogenic. This involves understanding the metabolic pathways of amino acids and their potential to contribute to both glucose and ketone body production.

**Why Each Wrong Option is Incorrect**
**Option A:** Typically, amino acids that are purely glucogenic are not ketogenic.
**Option B:** Similarly, purely ketogenic amino acids do not contribute to gluconeogenesis.
**Option D:** This option might represent an amino acid that does not fit the criteria of being both glucogenic (through the TCA cycle for gluconeogenesis) and ketogenic.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that certain amino acids can be both glucogenic and ketogenic, meaning their metabolism can lead to the production of both glucose (via gluconeogenesis) and ketone bodies.

**Correct Answer:** D. Leucine