**Core Concept**
Cytomegalovirus (CMV) infection is a common complication after organ transplantation due to the immunosuppressed state of the recipient. This allows latent CMV to reactivate or be reinfected from the donor organ.

**Why the Correct Answer is Right**
CMV infection occurs in up to 80% of transplant recipients, particularly those receiving organs from seropositive donors. The virus targets the endothelium, causing vascular damage and leading to complications such as graft dysfunction, pneumonia, and gastrointestinal disease. CMV can also reactivate from latent infection in the recipient, which is why prophylaxis or preemptive therapy is often employed.

**Why Each Wrong Option is Incorrect**
**Option A:** Herpes simplex virus (HSV) is also a concern in immunocompromised patients, but it is less common than CMV in organ transplant recipients. HSV typically causes mucocutaneous lesions rather than systemic disease.
**Option B:** Epstein-Barr virus (EBV) causes infectious mononucleosis and is associated with post-transplant lymphoproliferative disorder (PTLD), but it is not as common as CMV in organ transplant recipients.
**Option C:** Varicella-zoster virus (VZV) can cause shingles in immunocompromised patients, but it is not as frequently encountered as CMV in the post-transplant period.

**Clinical Pearl / High-Yield Fact**
CMV infection can be prevented or treated with antiviral medications such as valganciclovir, which should be initiated in high-risk patients. CMV prophylaxis is typically given for 3-6 months after transplantation.

**Correct Answer:** C.