Final answer:
For a patient with AIDS who develops P. jiroveci pneumonia, the nursing diagnosis of 'Impaired gas exchange' holds the highest priority because of the life-threatening impact on the lungs.
Step-by-step explanation:
A patient with acquired immunodeficiency syndrome (AIDS) develops P. jiroveci pneumonia. Among the nursing diagnoses presented, Impaired gas exchange has the highest priority for this patient. Due to the direct impact P. jiroveci pneumonia has on the lungs, causing difficulty in breathing and reducing the efficiency of oxygen-carbon dioxide exchange, this diagnosis takes precedence over other potential issues such as impaired oral mucous membranes, imbalanced nutrition, and activity intolerance. The patient's ability to breathe effectively is critical for survival and warrants immediate attention and care.