By far the most likely cause of the oliguria observed in this patient is hypovolemia. Volume status would be best assessed by floating a Swan-Ganz catheter to measure the preload pressures in the left atrium (by inference from the pulmonary capillary wedge pressures). Patients who have had a leaking aneurysm and then a long, usually difficult operation with large surgical fields that collect "third-space" fluids may be intravascularly depleted despite large volumes of intravenous fluid and blood replacement. The proper management usually involves titrating the cardiac output by providing as much fluid as necessary to keep the wedge pressures near 15 mm Hg. The other studies listed might become useful if urinary flow remains depressed after optimal cardiac output has been achieved, but in view of the probability of hypovolemia, they are not indicated as a first diagnostic study.