This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Nucleotides, Subject name: Biochemistry
In humans, rate limiting step of de novo pyrimidine synthesis is: 
Select and explain. 
A. Aspaate tanscarbamoylase, B. Carbamoyl phosphate synthetase-II, C. Dihydro orotate dehydrogenase, D. Ornithine tanscarbamoylase
B: Carbamoyl phosphate synthetase-II
Answer: B: Carbamoyl phosphate synthetase-II 
Explanation: De-novo pyramidine synthesis Rate limiting step in mammals CPS II in bacteria Aspaate transcarbamoylase CPS I CPS II Rate-limiting enzyme of urea cycle Rate limiting step of pyrimidine synthesis Found in mitochondria Found in cytosol Uses ammonia as nitrogen source Uses glutamine as nitrogen source Requires N-acetylglutamate as positive allosteric activator Doesnt require N-acetylglutamate Hypoxanthine-guanine phosphoribosyltransferase(HP) catalyzes salvage of the purine bases guanine and hypoxanthine into GMP and IMP.