<prompt> A 63-year-old man who has been previously healthy is admitted to the hospital with a 2-day history of cough, rigors, fever, and right-sided pleuritic chest pain. Chest x-ray shows consolidation of the right lower lobe RLL and a free-flowing right pleural effusion. Thoracentesis is performed, and the pleural fluid has the following characteristics:Cell count1110mm3Glucose75 mgdL serum glucose85 mgdLProtein4.0 gdL serum protein7.0 gdLLDH400 ULserum LDH200 UL, normal100-200 ULpH7.35What is the pathogenesis of the pleural effusion?Increase in hydrostatic pressureDecrease in oncotic pressureIncreased permeability of visceral pleural membrane capillariesBacterial infection in the pleural space  <response>: Clinical conditions associated with either an increase in hydrostatic pressure such as congestive heart failure or a decrease in oncotic pressure such as nephrotic syndrome are associated with transudative pleural effusions. This patients pleural fluid is exudative by all three of the Light criteria: pleural fluidserum protein ratio is greater than 0.5, pleural fluid LDHserum LDH ratio is greater than 0.6, and pleural fluid LDH is greater than two-thirds the upper limits of the normal serum LDH. The most likely explanation for an exudative pleural effusion in the setting of an acute pneumonia is a parapneumonic effusion. Parapneumonic effusions occur in about 40 of patients with bacterial pneumonia. Parapneumonic effusions are exudative due to the fact that there is increased permeability of the visceral pleural membrane capillaries, and interstitial fluid moves across the visceral pleura into the pleural space. Parapneumonic effusions may be simple or complicated. Simple parapneumonic effusions are sterile and free flowing. If bacteria invade the pleural space, neutrophils move into the pleural space and anaerobic metabolism of glucose results in a low pleural fluid pH 7.20 and glucose 60. The characteristics of this patients pleural fluid suggest that bacterial invasion of the pleural space has not occurred, and that this is a simple parapneumonic effusion. Hemorrhage into the pleural space occurs with trauma, cancer, and pulmonary embolism, but rarely with pneumonia. 