A 64-year-old man presents with symptoms of malaise, shortness of breath, edema, and no urine output for 24 hours. His past medical history is not significant, and his only medication is daily aspirin. On examination his JVP is 4 cm, heart sounds are normal, lungs are clear, and the abdomen is soft. A Foley catheter is inserted into his bladder for 200 cc of urine, which is sent for urinalysis. His urine output still remains low. Which of the following is the most appropriate initial diagnostic test?