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Pulmonary edema is primarily caused by the accumulation of fluid in the lungs, which can occur due to several mechanisms, with excessive pressure and increased permeability of the microvasculature being particularly significant. When there is heightened hydrostatic pressure in the pulmonary capillaries, it can lead to fluid being pushed out of the blood vessels and into the alveolar spaces, resulting in edema.

Furthermore, increased permeability of the microvasculature allows proteins and fluids to leak from the blood vessels into the interstitial space and alveoli, which contributes to the fluid buildup. This condition can arise from various underlying issues such as heart failure, where the heart’s inability to pump effectively raises pressure in the pulmonary circulatory system, or from inflammatory processes that damage the capillary walls, increasing their permeability.

The other options describe factors that do not directly lead to pulmonary edema. For instance, increased lung capacity and fluid retention do not directly address the mechanisms of fluid accumulation in the lungs. Similarly, decreased oxygen levels and lung inflammation are consequences or related conditions rather than primary mechanisms causing pulmonary edema, and poor circulation and infection can lead to secondary complications but do not define the direct physiological causes of edema in the lungs.