Final answer:
The primary reason for the prone positioning of an ARDS patient is to facilitate gas exchange by assisting with reexpansion of alveoli and preventing fluid diffusion into the interstitial space, which improves oxygenation.
Step-by-step explanation:
The primary reason a nurse would place a patient in the prone position during the early phases of acute respiratory distress syndrome (ARDS) is b. To assist with gas and nutrient exchange and to prevent the diffusion of fluid into the interstitial space. This practice can help improve oxygenation and facilitate easier breathing. Prone positioning may lead to better ventilation-perfusion matching by reexpanding collapsed alveoli predominantly present at the dorsal regions of the lungs when lying supine. Additionally, it can help redistribute pulmonary blood flow, effectively improving gas exchange and oxygenation in ARDS patients.