This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Nephrology, Subject name: Medicine
Investigations in a patient of oliguria revealed: Urine osmolality: 800 mOsm/kg. Urinary sodium 10 mmol/L.BUN: creatinine=20:1. What is the most likely diagnosis? 
Select and explain. 
A. Pre-renal acute renal failure, B. Acute tubular necrosis, C. Acute coical necrosis, D. Urinary tract obstruction
A: Pre-renal acute renal failure
Answer: A: Pre-renal acute renal failure 
Explanation: Pre renal Renal (ATN) Post renal Urine osmolality > 500 mOsm ~ 300 mOsm <400 mOsm Urine sodium <20 mmol/dl >20 mmol/dl Variable FeNa <1 >1 Variable Urine microscopic findings Hyaline casts Muddy brown casts Normal or RBC/WBC BUN: creatinine ratio >20:1 (normal 10:1) <20:1 >20:1