## Core Concept
The urea cycle, also known as the ornithine cycle, is a cycle of biochemical reactions that occurs in many animals, where it plays a crucial role in removing excess nitrogen from the organism. The cycle takes place primarily in the liver and, to a lesser extent, in the kidneys. The key steps involve the conversion of ammonia into urea through a series of reactions.

## Why the Correct Answer is Right
The correct answer, **C. Urea and ornithine**, is right because in the final step of the urea cycle, arginine is hydrolyzed by the enzyme **arginase** to form **urea** and **ornithine**. Urea is then excreted by the kidneys, while ornithine is recycled back into the cycle to combine with carbamoyl phosphate, continuing the process.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because fumarate and argininosuccinate are indeed intermediates in the urea cycle but are not the direct products of arginine hydrolysis.
- **Option B:** This option is incorrect because citrulline and aspartate are also intermediates in the urea cycle; however, they are not formed from the hydrolysis of arginine.
- **Option D:** This option is incorrect because while proline is an amino acid, it is not directly related to the hydrolysis products of arginine in the urea cycle.

## Clinical Pearl / High-Yield Fact
A key point to remember is that defects in the enzymes of the urea cycle can lead to hyperammonemia, a condition characterized by elevated levels of ammonia in the blood, which is toxic to the nervous system. A classic example is **carbamoyl phosphate synthetase I deficiency**, which leads to a severe form of hyperammonemia.

## Correct Answer: C. Urea and ornithine