Final answer:
Cytomegalovirus (CMV) causes nephritis in patients under immunosuppressive therapy post-kidney transplant, affecting a significant percentage of these patients.
Step-by-step explanation:
The organism that causes nephritis in patients receiving immunosuppressive agents after a kidney transplant is Cytomegalovirus (CMV). CMV can lead to serious outcomes in immunosuppressed individuals, such as transplant recipients. Around 60% of transplant recipients contract CMV infection, and over 20% develop symptomatic disease, which could include nephritis. While it may be due to CMV-contaminated tissues, it's typically caused by the reactivation of latent CMV due to the immunosuppressants necessary for transplantation. Symptoms like fever and leukopenia due to viremia can signal the presence of CMV infection.