A client with Pneumocystis jiroveci infection receiving pentamidine develops a fever. What does this likely indicate?

The presence of a fever in a client receiving pentamidine for a Pneumocystis jiroveci infection strongly suggests the possibility of another infection, particularly considering the leukopenic effects associated with pentamidine. Pentamidine can cause a reduction in white blood cell counts, increasing the client’s vulnerability to opportunistic infections. Fever is a common symptom that can indicate an infection, especially in an immunocompromised patient.

Thus, while the other factors mentioned could also be relevant in assessing the client's condition, the development of fever in the context of leukopenia renders option D the most likely explanation. It highlights the importance of continuous monitoring for febrile responses in patients undergoing treatment that affects their immune status, as they may not be able to effectively fight off additional infections.