Final answer:
Pneumonia and pulmonary edema increase the thickness of the gas exchange barrier due to increased capillary permeability, leading to fluid accumulation in the lung tissue.
Step-by-step explanation:
The question is asking about why pneumonia and pulmonary edema both increase the thickness of the barrier in gas exchange. The answer is B) Increased capillary permeability. Pneumonia and pulmonary edema lead to fluid accumulation in and around the alveoli which increases the thickness of the gas exchange barrier. This occurs due to increased capillary permeability, allowing fluid to leak from the capillaries into the alveoli and interstitial spaces. Other options such as decreased surfactant production (A), constriction of bronchioles (C), and reduced alveolar surface area (D) do not directly increase the thickness of the gas exchange barrier but could contribute to impaired lung function in other ways.