Pleural effusions are classified as transudative or exudative using Light's criteria, which remain the gold standard for differentiation since 1972. Transudative effusions result from systemic factors that alter hydrostatic or oncotic pressure balance across intact pleural membranes: heart failure (most common cause — elevated pulmonary capillary wedge pressure), cirrhosis with hepatic hydrothorax (peritoneal fluid tracking through diaphragmatic defects), and nephrotic syndrome (hypoalbuminemia reducing plasma oncotic pressure). Exudative effusions result from local factors that increase pleural membrane permeability or impair lymphatic drainage: pneumonia/parapneumonic effusion (most common cause of exudative effusion), malignancy (lung, breast, lymphoma), pulmonary embolism, tuberculosis, and autoimmune disease. Light's criteria classify an effusion as EXUDATIVE if any ONE of three criteria is met: (1) pleural fluid protein/serum protein ratio >0.5, (2) pleural fluid LDH/serum LDH ratio >0.6, (3) pleural fluid LDH >2/3 the upper limit of normal serum LDH. Light's criteria have 98% sensitivity for exudates but may misclassify some transudates as exudates (especially in heart failure patients on diuretics — serum protein becomes concentrated). In these cases, the serum-to-pleural fluid albumin gradient >1.2 g/dL correctly reclassifies most as...