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: Medicine
Which differentiating prerenal azotemia with ATN features favouring pre-renal 
Select and explain. 
A. Urine osmolality > 500 mosmol/kg, B. Sodium spot excretion < 10 ml/L, C. Plasma transferrin/Ig ratio, D. none
A: Urine osmolality > 500 mosmol/kg
Answer: A: Urine osmolality > 500 mosmol/kg 
Explanation: The normal 24-hour urine osmolality is, on average, 500-800 mOsmol/kg of water.
After 12-14 hours of fluid intake restriction, the urine osmolality should exceed 850 mOsmol/kg of water