In prerenal AKI, which combination best describes expected urinary indices?

• A. FeNa >2%, urine Na >40, low urine osmolality.
• B. FeNa <1%, urine Na <20, urine osmolality >500.
• C. FeNa >1%, urine Na 20-40.
• D. FeNa <1%, urine Na >40.

Answer: (B)

When renal perfusion drops, the kidney responds by increasing reabsorption of sodium and water to preserve blood flow and volume. That means the urine becomes highly concentrated and sodium loss is minimized.

So the expected urinary profile in prerenal AKI is a very low fractional excretion of sodium (FeNa < 1%), low urine sodium (typically < 20 mEq/L), and a high urine osmolality (often > 500 mOsm/kg). This pattern reflects intact tubular function with avid reabsorption due to hypoperfusion.

Why this pattern fits best: a low FeNa shows the kidney is holding onto sodium, not wasting it, because perfusion is reduced but the tubules are still working to reclaim Na and water. The low urine Na corroborates that little sodium is being excreted, and the high osmolality indicates concentrated urine from water reabsorption.

Patterns that don’t fit prerenal AKI help distinguish from intrinsic renal injury like ATN, where damage impairs tubule reabsorption, leading to higher FeNa (often >2%), higher urine Na (>40), and dilute urine with lower osmolality. A scenario with FeNa < 1% but high urine Na (>40) or with FeNa > 1% and urine Na 20–40 would also be inconsistent with prerenal physiology.