This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Medicine
Which investigation is known as the rule out test in massive pulmonary embolism? 
Select and explain. 
A. ECG, B. Echocardiogram, C. D-dimer, D. Aerial blood gas
C: D-dimer
Answer: C: D-dimer 
Explanation: The quantitative plasma d-dimer rises in the presence of DVT or PE because of the breakdown of fibrin by plasmin. The sensitivity of the d-dimer is >80% for DVT and >95% for PE. The d-dimer is a useful "rule out" test. More than 95% of patients with a normal (<500 ng/mL) d-dimer do not have PE. ECG: The most frequently cited abnormality, in addition to sinus tachycardia, is the S1Q3T3 sign. This finding is relatively specific but insensitive. Perhaps the most common abnormality is T-wave inversion in leads V1 to V4. Echocardiography is not a reliable diagnostic imaging tool for acute PE. Aerial blood gases lack diagnostic utility for PE, even though both PO2 and Pco2 often decrease. Ref: Harrisons Principles of Medicine, 18th Edition, Chapter 262, Pages 2172-73