Which compound is produced initially in the urea cycle?

The urea cycle, also known as the ornithine cycle, is crucial for the detoxification of ammonia in the body by converting it into urea for excretion. The first compound produced in this cycle is carbamoyl phosphate. This compound is synthesized in the mitochondria from ammonia (NH₃) and bicarbonate (HCO₃⁻), catalyzed by the enzyme carbamoyl phosphate synthetase I.

Carbamoyl phosphate then combines with ornithine to form citrulline, which is the subsequent step in the cycle. The formation of carbamoyl phosphate is essential because it serves as the starting substrate for the urea cycle, allowing for the continuation of the process of converting toxic ammonia into less harmful urea.

In the context of the other options, while ornithine and citrulline are important intermediates in the urea cycle, they are not the initial products of the pathway. Urea is the final product that is formed after a series of reactions in the cycle. Thus, recognizing carbamoyl phosphate as the initial compound produced in the urea cycle is critical to understanding the overall process of nitrogen metabolism.