A 63-year-old woman presents to the emergency room with symptoms of sudden onset of shortness of breath. She reports no chest discomfort, cough, sputum, or fever. Her past medical history is negative for any prior heart or lung problems. She was recently diagnosed with breast cancer and is undergoing active treatment. On examination, her blood pressure is 120/80 mm Hg, pulse 100/min, and heart and lungs are normal. There are no clinical signs of deep venous thrombosis (DVT). Which of the following investigations is most likely to rule out a pulmonary embolism (PE)?