How many molecules of ATP are used to produce one molecule of urea during ornithine cycle?

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The ornithine cycle (urea cycle) involves the following ATP-consuming steps:

1. Formation of carbamoyl phosphate from ammonia and carbon dioxide requires $2$ molecules of ATP.

2. Formation of argininosuccinate from citrulline and aspartate requires $1$ molecule of ATP (which is cleaved into AMP and pyrophosphate).

Thus, a total of $3$ molecules of ATP are used to produce one molecule of urea, although $4$ high-energy phosphate bonds are consumed.

Answer: $3$

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