A 70-year-old smoker presents with weight loss, cough, and a new, large, unilateral right-sided pleural effusion on chest X-ray. There is no evidence of cardiomegaly or pulmonary edema. Which of the following is the most likely underlying cause of this pleural effusion?

Explanation

The clinical context of an elderly smoker with weight loss and a new, large, unilateral pleural effusion, without signs of systemic fluid overload (like cardiomegaly or pulmonary edema seen in CHF, or ascites in cirrhosis, or anasarca in nephrotic syndrome), strongly suggests a malignant etiology. Malignant pleural effusions are typically exudative and often unilateral. While CHF, cirrhosis, and nephrotic syndrome can cause pleural effusions, they are usually transudative, often bilateral (CHF, nephrotic syndrome), or associated with other systemic signs (ascites in cirrhosis). The history of smoking and weight loss further points towards malignancy.