A 47-year-old man with alcoholic cirrhosis, listed for liver transplantation, presents with confusion. His medications include spironolactone 100 mg daily and furosemide 80 mg daily. On examination he is afebrile, confused with a blood pressure of 99/73 mmHg. His abdomen is distended with a fluid wave. Laboratory tests one week ago showed a serum creatinine of 1.1 mg/dL (97 mmol/L). At that time, an abdominal ultrasound showed ascites, a patent portal vein, and no evidence of liver cancer. Laboratory tests in the emergency department show a serum creatinine of 2.6 mg/dL (229 mmol/L), and a potassium of 4.9 mEq/L. A diagnostic paracentesis reveals 550 neutrophils/mm3. Urine output in the emergency department is 10 ml/hr. Urinalysis shows 75 RBC/hpf with casts.

In addition to withdrawal of diuretics and initiation of antibiotics, what is the next best step in the management of this patient?