<s>[INST] context: Biosynthesis of urea occurs in five steps.

1) Carbamoyl phosphate synthase-I (CPS-I), a mitochondrial enzyme, catalyzes the formation of carbamoyl phosphate by condensation of CO2 and ammonia.
Two molecules of ATP are required for the reaction. CPS-I is the rate-limiting enzyme of the urea cycle. It is an allosteric enzyme and allosterically activated by N-acetyl glutamate.
2) Ornithine transcarbamylase catalyzes the formation of citrulline from carbamoyl phosphate and ornithine.
3) Argininosuccinate synthase catalyzes the formation of argininosuccinate from citrulline and aspartate.
This reaction requires 1ATP, but 2 high energy phosphate bonds are consumed as ATP is converted to AMP + PPi.
The amino group of aspartate provides one of the two nitrogen atoms that appear in urea (The other one is provided by ammonia NH4).
4) Argininosuccinate lyase (argininosuccinase) catalyzes the cleavage of argininosuccinate into arginine and fumarate.
Fumarate enters in TCA cycle. 
5) Arginase catalyzes the formation of urea from arginine by hydrolytic cleavage of arginine to yield urea and ornithine.
Ornithine is thus regenerated and can enter mitochondria to initiate another round of the urea cycle. [/INST] question: Not a metabolic product of urea cycle- | options: A: Citrulline B: Ornithine C: Alanine D: Arginine | answer: C </s>