Best option: C

Reasoning: RIFLE stratifies AKI only by changes in serum creatinine/GFR or urine output, whereas KDIGO endorses the use of structural tubular-injury biomarkers—most prominently urinary NGAL—to help distinguish intrinsic (tubular) injury from functional (pre-renal) azotemia. Thus NGAL is a differentiating element in KDIGO but not in the original RIFLE scheme [McCullough 2013, PMID 23689653; Singer 2011, PMID 21412214; Ronco 2014, PMID 25672254].