## Core Concept
The differentiation between prerenal azotemia and Acute Tubular Necrosis (ATN) is crucial in understanding the cause of acute kidney injury (AKI). Prerenal azotemia is a reversible condition caused by decreased blood flow to the kidneys, while ATN is a more severe condition involving tubular damage. Key features help distinguish between these two conditions.

## Why the Correct Answer is Right
The correct answer, **B. Fractional excretion of sodium <1%**, is a key feature favouring prerenal azotemia. In prerenal azotemia, the kidneys avidly reabsorb sodium and water to correct the perceived hypovolemia or decreased effective circulating volume. This results in a low fractional excretion of sodium (FeNa) of less than 1%. The FeNa is calculated as (urine sodium / serum sodium) / (urine creatinine / serum creatinine) * 100. A FeNa 2%, presence of muddy casts, or a higher urine sodium level.
- **Option C:** While not specified, options like "urine osmolality > 500 mOsm/kg" could be considered more indicative of prerenal azotemia as well, but the specificity and sensitivity of such a value might not be as clear-cut as FeNa 2%, presence of renal tubular damage markers, or specific gravity close to 1.010), it would not favour prerenal azotemia.

## Clinical Pearl / High-Yield Fact
A crucial clinical pearl is that a **FeNa 2%** is more indicative of ATN. This differentiation is vital for guiding management, as prerenal azotemia often responds to fluid resuscitation, whereas ATN may require more supportive care and sometimes specific interventions.

## Correct Answer Line
**Correct Answer: B.**