This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Miscellaneous, Subject name: Anaesthesia
A 43-year-old trauma patient develops acute respiratory distress syndrome (ARDS) and has difficulty oxygenating despite increased concentrations of inspired O2 . After the positive endexpiratory pressure (PEEP) is increased, the patient's oxygenation improves. What is the mechanism by which this occurs? 
Select and explain. 
A. Decreasing dead-space ventilation, B. Decreasing the minute ventilation requirement, C. Increasing tidal volume, D. Increasing functional residual capacity
D: Increasing functional residual capacity
Answer: D: Increasing functional residual capacity 
Explanation: PEEP improves oxygenation by increasing functional residual capacity by keeping the alveoli open at the end of expiration. Extravascular lung water is shifted from the alveolar to the interstitial space. The overall result is to increase surface area for diffusive exchange of gases. Potential negative effects of increased PEEP include alveolar overdistention resulting in barotraumas (pneumothoraces), decreased venous return and decreased cardiac output, and increased minute ventilation requirements due to increased dead-space ventilation.