Common theme for USMLE questions is asking us to differentiate renal from prerenal causes of renal failure.
Always think that in prerenal there will be increased ADH trying to conserve water (and consequently Sodium)
Here are some important parameters:
- Fractional Excretion of Sodium <1% in prerenal and >2% in renal
- Serum urea / Serum Creatinine >20 in prerenal and <15 in renal (think dehydration in prerenal which has caused the higher urea concentration)
- Urine osmolality >500 mOsmol/kg in prerenal and <350 mOsmol/kg in renal