## Core Concept
The urea cycle, also known as the ornithine cycle, is a critical process by which the body converts ammonia into urea, which is then excreted. This cycle primarily occurs in the liver and is crucial for detoxifying ammonia, a byproduct of protein metabolism.

## Why the Correct Answer is Right
The correct answer, **C. Both (Liver and Kidney)**, is accurate because while the liver is the primary site where the urea cycle takes place, the kidneys also play a role, particularly in the regulation and excretion of urea. The liver converts ammonia into urea through a series of enzyme-catalyzed reactions, and the kidneys then filter urea from the blood and excrete it in urine. However, the key enzymes of the urea cycle are predominantly found in the liver.

## Why Each Wrong Option is Incorrect
- **Option A: Liver** is partially correct but incomplete because it does not acknowledge the role of the kidneys.
- **Option B: Kidney** is incorrect because, although the kidneys are involved in the excretion of urea, they are not the primary site of urea synthesis.
- **Option D: Brain** is incorrect because the brain is not a site for the urea cycle; instead, it is sensitive to ammonia toxicity.

## Clinical Pearl / High-Yield Fact
A crucial point to remember is that deficiencies in any of the enzymes of the urea cycle can lead to hyperammonemia, a condition characterized by elevated levels of ammonia in the blood, which can be life-threatening. This condition often presents in neonates and can lead to severe neurological symptoms.

## Correct Answer: C. Both (Liver and Kidney)