The most likely diagnosis is mesothelioma. Malignant pleural mesothelioma is a very aggressive tumor that arises from the mesothelial cells of the pleura. Patients most commonly present with symptoms of a slowly enlarging pleural effusion. This patient's physical examination findings (decreased breath sounds, dullness to percussion, decreased tactile fremitus, and no egophony) are consistent with a pleural effusion. The absence of an effusion on this patient's previous screening CT 5 months ago suggests that the effusion developed since then and correlates with development of his symptoms. His previous chest imaging also showed a calcified pleural plaque (arrows) that confirms his exposure to asbestos, which likely occurred when he worked as a plumber and is a strong risk factor for mesothelioma. While tobacco use has not been definitively identified as a risk factor for development of mesothelioma, patients with asbestos exposure have a dramatically increased risk for development of asbestos-related lung cancer and should be counseled to stop smoking. This combination of history, physical examination, and previous radiographic findings makes mesothelioma the most likely cause of this patient's newly diagnosed pleural effusion.

There are many other causes of a pleural effusion, including heart failure. However, this is unlikely in this patient because he has no evidence of volume overload on examination.

Although lymphoma can result in pleural effusions, the patient has no constitutional symptoms or other signs suggestive of lymphoma. Isolated pleural lymphoma is incredibly rare and is much less likely than mesothelioma in this patient.

Both pneumonia (areas of consolidation) and pleural effusion can result in decreased breath sounds and dullness to percussion on physical examination. However, consolidation would cause increased, rather than decreased, tactile fremitus, and egophony would be present.