<s>[INST] context: The question indirectly asks most common viral infection post transplant . The risk of viral infection is highest during the first 6 months after transplantation and the most common problem is CMV infection. CMV disease may arise because of either reactivation of latent infection or primary infection that can be transmitted by an organ from a CMV-positive donor. Recipients at most risk from CMV infection are those who are CMV seronegative and receive an organ from a CMV-seropositive donor (about half of all UK donors are CMV seropositive). CMV disease typically presents at 4-8 weeks with a high swinging fever, lethargy and leukopenia. The severity of the disease is variable and the clinical picture depends on the organ system most affected. It may present as: pneumonia , gastrointestinal disease , hepatitis , retinitis , encephalitis. Severe CMV disease is potentially fatal. Prophylaxis for CMV consists of administration of antiviral agents but less effective alternatives. A diagnosis of active CMV infection is confirmed by PCR to detect viral DNA in whole blood, and by histological examination of biopsy material.Treatment is with antiviral agents ref : Bailey 27th ed , chapter 82 , pg no : 1541 [/INST] question: A person with renal transplant developed infection of graft within 2 month. Most probable cause of infection is ? | options: A: Polyoma BK virus B: CMV C: Hep. C D: Herpes simples | answer: B </s>