**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, which is then excreted. This process is essential for detoxifying ammonia, a toxic byproduct of protein metabolism.

**Why the Correct Answer is Right**
The urea cycle involves the sequential conversion of ammonia to carbamoyl phosphate, which is then converted to citrulline, argininosuccinate, arginine, and finally urea. This process occurs in the mitochondria of liver cells, where the necessary enzymes and substrates are present. The urea cycle is crucial for maintaining nitrogen balance in the body and preventing ammonia toxicity.

**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect because the citric acid cycle (also known as the Krebs cycle or tricarboxylic acid cycle) occurs in the mitochondria but is not directly involved in the urea cycle.
**Option B:** This is incorrect because glycolysis occurs in the cytosol of liver cells, not in the mitochondria where the urea cycle takes place.
**Option C:** This is incorrect because protein synthesis occurs on ribosomes in the cytosol, not in the mitochondria where the urea cycle occurs.

**Clinical Pearl / High-Yield Fact**
The urea cycle is a critical process that helps maintain nitrogen balance in the body. In conditions such as liver disease or urea cycle disorders, ammonia toxicity can occur, leading to neurological symptoms and even death.

**Correct Answer: C. Mitochondria of liver cells.**