Investigations in a patients of oliguria revealed; Urine osmolality:620 mosm/kg.Urinary sodium:12 mmol/L. Urine/plasma urea ratio: 13:3. The most likely diagnosis is -

• A. Prerenal acute renal failure
• B. Acute tubular necrosis
• C. Acute cortical necrosis
• D. Urinary trat obstruction