Light's criteria help distinguish exudative from transudative pleural effusions; which condition is the most common cause of transudates?

• A. Pneumonia
• B. Malignancy
• C. Congestive heart failure
• D. Pulmonary embolism

Answer: (C)

Transudative pleural effusions come from systemic fluid balance problems, specifically changes in hydrostatic or oncotic pressures, rather than local pleural inflammation. The most common cause is congestive heart failure, because the elevated pressure in the pulmonary and systemic venous system pushes fluid into the pleural space, typically producing bilateral, basilar effusions that are low in protein and LDH.

Pneumonia tends to cause pleural fluid from local infection and inflammation, which characterizes an exudate with higher protein and LDH. Malignancy usually yields an exudative effusion due to tumor involvement and inflammatory mediators. Pulmonary embolism can cause an exudate from inflammation, though small transudates aren’t the typical presentation.