Which organism is associated with pneumonia after travel in the southwestern United States?

The organism associated with pneumonia following travel in the southwestern United States is Coccidioides immitis, which is a fungus that causes coccidioidomycosis, commonly known as valley fever. This condition is endemic in certain regions, particularly in the southwestern U.S., where the spores of the fungus are present in the soil.

Coccidioidomycosis can lead to pneumonia-like symptoms after inhalation of the spores, especially in individuals who have recently traveled to areas where the fungus is prevalent. It can range from mild illness to more severe respiratory infections, particularly in immunocompromised individuals or those who are pregnant.

Other organisms listed, such as Burkholderia cepacia, Bacillus anthracis, and Pseudomonas aeruginosa, are either associated with different geographical locations, other types of infections, or specific risk factors that do not align with the classic presentation of pneumonia after travel to the southwestern U.S. Thus, the association of Coccidioides immitis with respiratory illness in this context makes it the correct answer.