This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Biochemistry
The enzyme carbamoyl phosphate synthetase requires: 
Select and explain. 
A. Mg++, B. Ca++, C. K+, D. None of the above
A: Mg++
Answer: A: Mg++ 
Explanation: One molecule of ammonia condenses with CO2 in the presence of two molecules of ATP to form carbamoyl phosphate. The reaction is catalyzed by the mitochondrial enzyme carbamoyl phosphate synthetase-I (CPS-I).
Reference: Vasudevan 7th  ed, pg 203