**Core Concept**
N-acetylglutamate synthase (NAGS) is an enzyme involved in the synthesis of N-acetylglutamate, an allosteric activator of carbamoyl phosphate synthetase I (CPSI), the first committed step in the urea cycle. The allosteric activation of CPSI by N-acetylglutamate is crucial for regulating the urea cycle, particularly in response to changes in nitrogen availability.

**Why the Correct Answer is Right**
The allosteric activator of NAG synthase is glutamate, which binds to a specific site on the enzyme, inducing a conformational change that increases the enzyme's activity. This is an example of a positive allosteric modulation, where the binding of a ligand increases the activity of the enzyme. The increased activity of NAG synthase leads to an increase in N-acetylglutamate production, which in turn activates CPSI, initiating the urea cycle.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because there is no known allosteric activator of NAG synthase in the literature.

**Option B:** This option is incorrect because glutamine is not an allosteric activator of NAG synthase. While glutamine is involved in the urea cycle, its primary role is as a substrate for carbamoyl phosphate synthetase II, not as an allosteric activator.

**Option C:** This option is incorrect because ammonia is a substrate for the enzyme, not an allosteric activator. Ammonia is converted to N-acetylglutamate by NAG synthase, but it does not activate the enzyme.

**Clinical Pearl / High-Yield Fact**
The urea cycle is a critical pathway for nitrogen excretion, particularly in the liver. Allosteric regulation of key enzymes, such as CPSI, by N-acetylglutamate ensures that the urea cycle is upregulated in response to increased nitrogen availability, preventing ammonia toxicity.

**Correct Answer:** B. Glutamate