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
A patient presents with acute gastroenteritis in a dehydrated state. Which of the following conditions is highly suggestive of pre-renal renal failure? 
Select and explain. 
A. Urinary sodium > 40 m mol; Urine osmolality <350m osm/L, B. Urinary sodium > 40 m mol; Urine osmolality > 500m osm/L, C. Urinary sodium < 20 m mol; Urine osmolality > 500m osm/L, D. Urinary sodium < 20 m mol; Urine osmolality < 350m osm/L
C: Urinary sodium < 20 m mol; Urine osmolality > 500m osm/L
Answer: C: Urinary sodium < 20 m mol; Urine osmolality > 500m osm/L 
Explanation: None