A client who had cardiac surgery 24 hours ago has a urine output averaging 19 ml/hr for 2 hours. The client received a single bolus of 500 ml of IV fluid. Urine output for the subsequent hour was 25 ml. Daily laboratory results indicate the blood urea nitrogen is 45 mg/dL and the serum creatinine is 2.2 mg/dL. A nurse interprets the client is at risk for: