Which statement correctly describes Light's criteria for pleural effusions?

• A. Transudates result from increased capillary permeability due to infection.
• B. Exudates are typically due to systemic factors and often bilateral.
• C. Transudates are associated with conditions like congestive heart failure and cirrhosis.
• D. Exudates are defined by fluid that is less protein-rich than serum.

Answer: (C)

Light's criteria distinguish pleural effusions by the protein and LDH content of the pleural fluid relative to serum, reflecting different underlying processes. Transudates arise from systemic factors that disrupt hydrostatic or oncotic pressures (such as congestive heart failure or cirrhosis), yielding fluid that is relatively low in protein and LDH. Exudates come from local inflammatory or malignant processes that increase vascular permeability or impair lymphatic drainage, producing higher protein content and higher LDH in the fluid. The statement that transudates are associated with congestive heart failure and cirrhosis is correct. The other statements mischaracterize the nature of transudates and exudates: infection-related changes are more about exudates, exudates are not typically described as arising from systemic factors or bilateral disease, and exudates are not defined by being less protein-rich.