A 55-year-old woman, who has an advanced breast cancer, suffers from a sudden onset of acute shortness of breath. It is associated with left sided chest pain that worsens with coughing and deep breathing. Her left arm is swollen and painful after central venous catheter placement for chemotherapy. Her BP 110/70, HR 130 regular, RR 28/min, and O2 saturation 87% on room air. CVS and chest exam is normal. The left arm is swollen from elbow to higher up. What is the single best test for evaluation of her shortness of breath?