This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Carbohydrates, Subject name: Biochemistry
Kreb's cycle and urea cycle are linked by- 
Select and explain. 
A. Succinate, B. Malate, C. a ketoglutarate, D. Fumarate
D: Fumarate
Answer: D: Fumarate 
Explanation: Ans. is 'd' i.e., Fumarate * Fumarate is released during urea cycle, which is an intermediate of Kreb's cycle, thus linking the two.Points to remember about urea cycle1. Urea cycle happens is a pathway active only in liver.2. It is partly mitochondrial, partly cytoplasmic.3. CPS I is the rate limiting enzyme of urea cycle and CPSI is stimulated by NAG (N Acetyl Glutamate) (Please note CPS II is an enzyme of pyrimidine synthesis).4. Net ATP used for detoxifying 1 molecule of ammonia is 1.55. N1 donor of urea is ammonia and N2 donor is aspartate.6. Arginase is the step where in urea is released.