Biochemistry Which one of the following occurs in the urea cycle? A Carbamoyl phosphate is derived directly from glutamine and CO2. B Ornithine reacts with aspartate to generate argininosuccinate. C The a-amino group of arginine forms one of the nitrogens of urea. D Ornithine directly reacts with carbamoyl phosphate to form citrulline. 💡 Explanation Which one of the following occurs in the urea cycle? ✓ Correct Answer: D. Ornithine directly reacts with carbamoyl phosphate to form citrulline.