This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Kidney, Subject name: Medicine
Fractional excretion of sodium (FENa) is calculated as 
Select and explain. 
A. x 100, B. x 100, C. x 100, D. x 100
D: x 100
Answer: D: x 100 
Explanation: The fractional excretion of sodium (FeNa) is the fraction of the filtered sodium load that is reabsorbed by the tubules FENa relates sodium clearance to creatinine clearance. As such, it depends on sodium intake, effective intravascular volume, GFR, diuretic intake, and intact tubular reabsorptive mechanisms. With prerenal azotemia, the FeNa may be below 1%, suggesting avid tubular sodium reabsorption. FENa may be >1.0% in prerenal ARF if patients are on diuretics or with pre-existing CKD, salt-wasting syndromes or adrenal insufficiency. Ref: Harrison 19e 1807