**Core Concept**
Beta-alanine is a non-essential amino acid that plays a crucial role in the synthesis of carnosine, a dipeptide found in muscle tissue. The formation of beta-alanine involves a series of biochemical reactions that ultimately lead to its production.

**Why the Correct Answer is Right**
Beta-alanine is formed through the breakdown of aspartate (aspartic acid) via the action of the enzyme aspartate aminotransferase (AAT). This enzyme catalyzes the transfer of an amino group from aspartate to alpha-ketoglutarate, resulting in the formation of glutamate and oxaloacetate. The oxaloacetate is then converted into beta-alanine through a series of reactions involving the enzyme aspartate oxidase and the enzyme alanine aminotransferase.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not accurately describe the biochemical pathway involved in the formation of beta-alanine.
* **Option B:** This option is incorrect because it suggests a different enzyme is responsible for the formation of beta-alanine, when in fact aspartate aminotransferase is the primary enzyme involved.
* **Option C:** This option is incorrect because it implies that beta-alanine is formed directly from the breakdown of another amino acid, when in fact it is formed through a series of reactions involving aspartate and other enzymes.

**Clinical Pearl / High-Yield Fact**
Beta-alanine supplementation is often used by athletes to increase muscle carnosine levels, which can help delay the onset of fatigue and improve exercise performance.

**Correct Answer: D. Aspartate is broken down by aspartate aminotransferase to form beta-alanine.