**Core Concept**
The urea cycle, also known as the ornithine cycle, is a series of biochemical reactions that occur in the liver to convert ammonia into urea for excretion. This process involves the sequential action of several enzymes, which are crucial for maintaining nitrogen balance in the body.

**Why the Correct Answer is Right**
The correct sequence of enzymes in the urea cycle is: Ornithine Transcarbamoylase (OTC) catalyzes the transfer of a carbamoyl group from carbamoyl phosphate to ornithine, forming citrulline. Arginosuccinate Synthetase then combines citrulline with aspartate to form arginosuccinate. Argininosuccinate Lyase breaks down arginosuccinate into arginine and fumarate. Finally, Arginase cleaves arginine into urea and ornithine, completing the cycle. This sequence ensures the efficient conversion of ammonia into urea for excretion.

**Why Each Wrong Option is Incorrect**
**Option A:** Arginase is the last enzyme in the urea cycle, not the first.
**Option B:** Arginosuccinate Synthetase is involved in the formation of arginosuccinate from citrulline and aspartate, but it is not the first enzyme in the cycle.
**Option C:** Argininosuccinate Lyase breaks down arginosuccinate, but it is not the first enzyme in the cycle.
**Option D:** Ornithine Transcarbamoylase is the first enzyme in the urea cycle, catalyzing the transfer of a carbamoyl group from carbamoyl phosphate to ornithine.

**Clinical Pearl / High-Yield Fact**
Deficiencies in enzymes of the urea cycle, such as Ornithine Transcarbamoylase (OTC) deficiency, can lead to hyperammonemia and neurological symptoms. Early recognition and treatment of these defects are crucial to prevent long-term neurological damage.

**Correct Answer:** D. Ornithine Transcarbamoylase