Final answer:
The patient with severe burns, a serum creatinine level of 5 mg/dL, and a GFR decrease of 75% is exhibiting the 'Failure' stage of acute kidney failure.
Step-by-step explanation:
The patient with a serum creatinine level of 5 mg/dL and a 75% decrease in glomerular filtration rate (GFR) is exhibiting failure as defined by the acute kidney failure classification consensus criteria. According to the information provided, the criteria for failure include a creatinine level that is 3.0 times the baseline or higher and/or a urine output below 0.3 ml/kg for 24 hours. Given the severe elevation in creatinine, this patient exceeds the threshold for the injury stage and falls within the definition of failure.