**Core Concept**
The urea cycle is a critical metabolic pathway that eliminates ammonia from the body by converting it into urea. Two nitrogen atoms are incorporated into the urea cycle, and their source is essential for understanding the cycle's regulation and potential disorders.

**Why the Correct Answer is Right**
The two nitrogen atoms that are incorporated into the urea cycle are derived from aspartate and ammonia. Aspartate donates one nitrogen atom, while ammonia donates the other. This process occurs in the liver, where aspartate aminotransferase (AST) catalyzes the transfer of an amino group from aspartate to α-ketoglutarate, producing oxaloacetate and glutamate. The ammonia molecule then combines with the carbamoyl phosphate to form citrulline, incorporating the second nitrogen atom.

**Why Each Wrong Option is Incorrect**
* **Option A:** Aspartate is indeed a source of one nitrogen atom, but it is not the only source.
* **Option B:** Ammonia is a source of one nitrogen atom, but it is not the only source.
* **Option D:** This option is incomplete and does not specify a correct source of nitrogen.

**Clinical Pearl / High-Yield Fact**
The urea cycle is regulated by feedback inhibition, where the end product, arginine, inhibits the enzyme argininosuccinate synthetase, preventing excessive urea production.

**Correct Answer:** C. Ammonia and Aspartate.