Which type of acute renal failure is indicated by a fractional excretion of sodium (FENa) less than 1%?

A fractional excretion of sodium (FENa) less than 1% is indicative of prerenal acute kidney injury (AKI). This condition typically occurs when there is insufficient blood flow to the kidneys, leading to a decreased glomerular filtration rate. In response to low perfusion, the kidneys conserve sodium to maintain circulatory volume, resulting in a low FENa.

In prerenal AKI, the kidneys are functioning properly, but they are not receiving adequate blood flow due to various reasons such as dehydration, heart failure, or septic shock. The body's regulatory mechanisms continue to function, focusing on sodium retention to increase intravascular volume and improve renal perfusion.

FENa values above 2% would suggest intrarenal causes, as injured renal tubules are unable to appropriately reabsorb sodium. Postrenal causes involve obstruction of urine flow, which may not be directly reflected in FENa until significant renal impairment has occurred. Functional AKI generally refers to conditions like acute tubular necrosis due to medications but isn't specifically represented by a low FENa in this context.