A 58-year-old man with cirrhosis complains of worsening fatigue and confusion over the past 5 days. He also reports that over the past 48 hours he has had a declining urinary output. On examination, he is gaunt and jaundiced. He has tense ascites and a liver span of 7 cm in the midclavicular line. Lab result reveal a WBC 4600/mm3, Hb 9.4 g/dL, and PCB 29%. BUN of 34 mg/dL and a creatinine of 3.1 mg/dL. A urinary Na <10 mEq/L. Most appropriate treatment for his elevated BUN and creatinine?