Which urinary indices are typical of prerenal AKI?

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

Answer: (C)

Prerenal AKI occurs when renal perfusion falls, so the kidneys conserve water and sodium to maintain effective circulating volume. That conservation drives a highly concentrated urine with very little sodium lost in the urine. The pattern you’d expect is a FeNa under 1%, a urine sodium under 20 mEq/L, and a urine osmolality well above 500 mOsm/kg. This combination shows the tubules are reabsorbing most filtrate to preserve volume, not leaking it.

In contrast, intrinsic renal injury like acute tubular necrosis or obstruction disrupts tubular function, so the kidney can’t reabsorb sodium or concentrate urine effectively. You’d see a FeNa >2%, urine Na >40 mEq/L, and urine osmolality around or below 350 mOsm/kg. Remember that diuretics can make FeNa less reliable, and FeUrea may be used in those cases.