Which differentiating prerenal azotemia with ATN features favoring pre-renal azotemia -a) Urine osmolality > 500 mosmol/kgb) Sodium spot excretion 1d) Plasma BUN/creatinine ratio < 20

**Core Concept**
Pre-renal azotemia and acute tubular necrosis (ATN) are two types of acute kidney injury (AKI) that can present with similar clinical and laboratory features. Differentiating between the two is crucial for appropriate management and prognosis.

**Why the Correct Answer is Right**
Pre-renal azotemia is characterized by increased urine osmolality (> 500 mosmol/kg) due to renal vasoconstriction and concentration of urine. This is in contrast to ATN, where urine osmolality is typically low ( 40 mEq/L) and a low plasma BUN/creatinine ratio ( 500 mosmol/kg is actually a characteristic of pre-renal azotemia, not a differentiating feature.
* **Option B:** This option is incorrect because a sodium spot excretion of 1 mEq/L is extremely low and not a characteristic of pre-renal azotemia or ATN.
* **Option C:** This option is incorrect because a plasma BUN/creatinine ratio > 20 is actually more indicative of ATN, not pre-renal azotemia.

**Clinical Pearl / High-Yield Fact**
In the setting of AKI, remember the "FENa" rule: a low fractional excretion of sodium ( 2%) suggests ATN.

**Correct Answer: D. Plasma BUN/creatinine ratio < 20**