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?

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