**Core Concept**
Carbamoyl phosphate synthase-I (CPS-I) is a key enzyme in the urea cycle, responsible for catalyzing the synthesis of carbamoyl phosphate from ammonia, bicarbonate, and ATP. This enzyme is primarily found in the liver and plays a crucial role in detoxifying ammonia by converting it into urea.

**Why the Correct Answer is Right**
The correct answer is **N-acetylglutamate**. N-acetylglutamate acts as an allosteric activator of CPS-I, thereby stimulating the enzyme's activity. This is essential for regulating the urea cycle in response to changes in amino acid levels, particularly arginine and aspartate. When N-acetylglutamate binds to CPS-I, it increases the enzyme's affinity for the substrate, allowing for efficient urea production. This regulatory mechanism is essential for maintaining ammonia homeostasis in the body.

**Why Each Wrong Option is Incorrect**
* **Option A:** is incorrect because it is not directly related to the regulation of CPS-I. While it may play a role in other metabolic pathways, it is not a known activator of this enzyme.
* **Option B:** is incorrect because it is a byproduct of the urea cycle, not a stimulator of CPS-I. It is produced later in the cycle, after CPS-I has already been activated.
* **Option C:** is incorrect because it is not a known regulator of CPS-I. While it may play a role in other aspects of metabolism, it is not directly involved in the activation of this enzyme.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that N-acetylglutamate is a critical regulator of the urea cycle, and its levels can be affected by various factors, including arginine deficiency, which can lead to hyperammonemia.

**Correct Answer:** D. N-acetylglutamate