A 15-month-old girl has a history of poor oral fluid intake, occasional vomiting, rapid breathing, and decreased urine output. Physical examination reveals a pulse of 150/min, BP of 120/80, and a respiratory rate of GO/min. There are bibasilar rales, and the liver is palpable. All procedures might be helpful in evaluating the oliguria EXCEPT
Giving a fluid challenge with isotonic saline, 20 ml/kg Determining the urine sodium concentration Determining the BUN and serum creatinine levels Giving a dose of intravenous furosemide