Urea is end product of
**Core Concept**
The question is testing the understanding of nitrogen metabolism, specifically the final step in the breakdown of amino acids. Urea is a key end product of this process, which involves the conversion of ammonia to a less toxic compound.
**Why the Correct Answer is Right**
In the liver, ammonia is converted to carbamoyl phosphate through the action of the enzyme carbamoyl phosphate synthetase I. Carbamoyl phosphate then reacts with ornithine to form citrulline, which is transported to the kidneys for further processing. In the kidneys, citrulline is converted to argininosuccinate by the enzyme argininosuccinate synthetase. Argininosuccinate is then cleaved to form arginine and fumarate. The enzyme arginase finally converts arginine to urea and ornithine. This process is known as the urea cycle.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the correct answer is related to the urea cycle, which is not the end product of glycolysis.
* **Option B:** This option is incorrect because glycogen is a storage form of glucose, not a nitrogen-containing compound.
* **Option C:** This option is incorrect because the citric acid cycle (also known as the Krebs cycle or tricarboxylic acid cycle) is involved in the breakdown of acetyl-CoA, not nitrogen-containing compounds.
**Clinical Pearl / High-Yield Fact**
The urea cycle is a critical process for removing excess nitrogen from the body, and disruptions in this pathway can lead to hyperammonemia, a life-threatening condition.
**Correct Answer:** D. Urea is the end product of the urea cycle.