## 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 involves the conversion of ammonia into urea, which is then excreted. A key step in this cycle involves the hydrolysis of arginine.

## Why the Correct Answer is Right
The correct answer, **C. Urea + Ornithine**, is accurate because in the urea cycle, arginine is hydrolyzed by the enzyme **arginase** to form **urea** and **ornithine**. This step is critical as it not only produces urea, which is the end product of the cycle and is excreted, but also regenerates ornithine, which is necessary for the cycle to continue. The reaction is as follows: Arginine + H2O → Urea + Ornithine.

## Why Each Wrong Option is Incorrect
- **Option A:** This option is incorrect because it inaccurately represents the products of arginine hydrolysis in the urea cycle. While citrulline and aspartate are indeed substrates that combine in earlier steps of the cycle, they are not the products of arginine hydrolysis.
- **Option B:** This option is incorrect because carbamoyl phosphate and ornithine are reactants in an early step of the urea cycle, not products of arginine hydrolysis.
- **Option D:** This option is incorrect because fumarate and argininosuccinate are intermediates in the urea cycle but are not the direct products of arginine hydrolysis.

## Clinical Pearl / High-Yield Fact
A memorable point for students is that defects in the urea cycle, including issues with arginase deficiency, can lead to hyperargininemia, a condition characterized by elevated levels of arginine in the blood and often associated with neurological symptoms. Understanding the key steps and enzymes of the urea cycle is crucial for recognizing and managing such disorders.

## Correct Answer: C. Urea + Ornithine