## **Core Concept**
Carbamoyl phosphate synthetase I (CPS I) is a crucial enzyme in the urea cycle, which is primarily located in the liver. This enzyme catalyzes the synthesis of carbamoyl phosphate from ammonia, carbon dioxide, and ATP. The urea cycle is vital for the disposal of excess nitrogen in the form of urea.

## **Why the Correct Answer is Right**
The correct answer, **C. Mitochondrial**, is right because carbamoyl phosphate synthetase I is indeed located in the mitochondria of liver cells. This enzyme plays a pivotal role in the urea cycle by converting ammonia into carbamoyl phosphate, which then reacts with ornithine to form citrulline. This process is critical for nitrogen detoxification and the production of urea for excretion.

## **Why Each Wrong Option is Incorrect**
- **Option A: Cytosolic** - This is incorrect because while there are cytosolic enzymes involved in the urea cycle (e.g., carbamoyl phosphate synthetase II, which is involved in pyrimidine synthesis), carbamoyl phosphate synthetase I is specifically located in the mitochondria.
- **Option B: Peroxisomal** - This is incorrect because peroxisomes are involved in the breakdown of fatty acids and amino acids, but they are not the site of carbamoyl phosphate synthetase I activity.
- **Option D: Lysosomal** - This is incorrect because lysosomes are primarily involved in digestion and recycling of cellular waste, not in the synthesis of carbamoyl phosphate.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that deficiencies in enzymes of the urea cycle, such as carbamoyl phosphate synthetase I, can lead to severe hyperammonemia, which is toxic to the nervous system. This condition requires immediate medical attention and can present with symptoms like vomiting, lethargy, and seizures.

## **Correct Answer:** . Mitochondrial