This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Acute Renal Failure, Subject name: Medicine
Decreased fractional excreion of sodium is caused by- 
Select and explain. 
A. Prerenal azotemia, B. Cortical ischemia, C. Glomerulonephritis, D. Renal tubular acidosis
A: Prerenal azotemia
Answer: A: Prerenal azotemia 
Explanation: Ans. is 'a' i.e., Prerenal azotemia Diagnostic IndexPrerenal ARFIntrinsic Renal ARFo Fractional Excretion of NaQ<1>1o trine Sodium ConcentrationQ (mmol/L)<10>20o Urine Creatinine to PlasmaQ Creatinine ratio>40<20o trine osmolalityQ (mosmol/kgH2O)>500-350o Renal failure IndexQ<1>1o Urine urea NitrogenQ to plasma urea Nitrogen>8<3o Plasma BUN/CreatinineQ Ratio>20<10-15o Urinary SedimentQHyaline CastsMuddy Brown Granular Casto Urine Specific gravityQ>1.020~1.010