This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Proteins and Amino Acids, Subject name: Biochemistry
The two nitrogen atoms, that are incorporated into the urea cycle. The source of the nitrogen are 
Select and explain. 
A. Glutamate & Aspartate, B. Aspartate & Arginine, C. Aspartate & NH3, D. Glutamate & NH3
C: Aspartate & NH3
Answer: C: Aspartate & NH3 
Explanation: (Aspartate & NH3) (448 - R S 7th) (251 - Lippincott's)UREA CYCLE* One nitrogen of the urea molecule is supplied by free NH3 and other nitrogen by aspartate* Glutamate is the immediate precursor of both ammonia through oxidative deamination by glutamate dehydrogenase) and aspartate nitrogen (through transmination of oxaloacetate by aspartate aminotransferase* Urea is produced by the hydrolysis of arginine* The urea cycle is linked to the TCA cycle through the production of fumarate. Amino acid catabolism is therefore directly coupled to energy production