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
Which substance enters the urea cycle and gets regenerated 
Select and explain. 
A. Ornithine, B. Aspartate, C. Citrulline, D. Fumarate
A: Ornithine
Answer: A: Ornithine 
Explanation: Hence, Urea cycle is also called Ornithine cycle.
Note: Fumarate does not enter the cycle, it is a by product of the cycle.