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

Despite the history of poor intake of oral fluid, this infant presents with signs of congestive heart failure and fluid overload. In view of these findings, a fluid challenge could be dangerous, and it is unlikely that she would respond. If the oliguria is the result of congestive heart failure and poor renal perfusion, the urine sodium concentration and fractional excretion of sodium should be low, the blood urea nitrogen and serum creatinine should be normal or slightly elevated, and the child may respond well to furosemide.