**Core Concept**
Leucine is a branched-chain amino acid (BCAA) that plays a crucial role in protein synthesis, metabolism, and cellular signaling. It is one of the essential amino acids, meaning it cannot be synthesized by the human body and must be obtained through the diet.

**Why the Correct Answer is Right**
Leucine acts as a key regulator of protein synthesis by activating the mechanistic target of rapamycin (mTOR) pathway, which is a central regulator of cell growth and proliferation. Leucine also has a unique structure with a branched side chain, which allows it to interact with specific enzymes and receptors. This interaction enables leucine to stimulate protein synthesis and inhibit protein degradation.

**Why Each Wrong Option is Incorrect**
* **Option A:** Leucine is not a non-essential amino acid. While it can be synthesized by some microorganisms, humans require dietary leucine.
* **Option B:** Leucine is not a glucogenic amino acid. It is actually a ketogenic amino acid, meaning it can be converted into acetyl-CoA and ketone bodies.
* **Option C:** Leucine is not a specific neurotransmitter or hormone. While it can interact with various signaling pathways, it is primarily known for its role in protein metabolism.

**Clinical Pearl / High-Yield Fact**
Leucine supplementation has been shown to be beneficial for patients with muscle wasting diseases, such as cancer cachexia, and can also help to promote muscle protein synthesis in healthy individuals.

**Correct Answer:** C.