Normal gas exchange requires matched ventilation and perfusion. Ventilation delivers oxygen to alveoli. Perfusion delivers blood to pulmonary capillaries. A well-matched lung region can transfer oxygen into blood and remove carbon dioxide. Mismatch becomes clinically important when one side of the system fails. Pulmonary embolism is the classic perfusion failure. A thrombus, often originating from deep venous thrombosis, lodges in the pulmonary arterial circulation. Distal alveoli may still receive air, but blood flow beyond the clot is reduced or absent. That pattern is different from many parenchymal lung diseases, where both ventilation and perfusion may be abnormal in the same region because diseased lung receives less air and less blood. The RN does not read the image, but this reasoning helps the nurse understand why the report must be interpreted with symptoms, pretest probability, chest radiograph or other imaging, D-dimer when applicable, and overall clinical risk. For NCLEX-RN (United States), items rarely announce the topic in the first sentence. Anchor to objective data, trajectory, and the safest next step for the role named in the stem before distractors compete. On...