**Core Concept**
Cytomegalovirus (CMV) is a common herpesvirus that can cause severe illness in immunocompromised individuals, particularly transplant recipients. CMV infection is a leading cause of morbidity and mortality in this population due to the impaired immune response.

**Why the Correct Answer is Right**
CMV infection occurs when the virus reactivates from latency in the host, exploiting the compromised immune system of transplant recipients. The virus can be transmitted through the transplanted organ, blood transfusions, or direct contact with infected bodily secretions. CMV infection can lead to a range of clinical manifestations, including fever, fatigue, and organ dysfunction. The impaired immune response in transplant recipients makes it difficult to control CMV replication, leading to severe and potentially life-threatening complications.

**Why Each Wrong Option is Incorrect**
**Option A:** Herpes simplex virus (HSV) is another common herpesvirus that can cause illness in immunocompromised individuals. However, it is less common than CMV in transplant recipients and typically presents with localized skin or mucosal lesions.
**Option B:** Varicella-zoster virus (VZV) can cause herpes zoster (shingles) in immunocompromised individuals, but it is not the most common infection in transplant recipients. VZV can also cause varicella (chickenpox) in susceptible individuals.
**Option C:** Epstein-Barr virus (EBV) is a common herpesvirus that can cause infectious mononucleosis in healthy individuals. While EBV can also infect immunocompromised individuals, it is not the most common infection in transplant recipients.

**Clinical Pearl / High-Yield Fact**
Transplant recipients are at increased risk of CMV infection due to the impaired immune response. CMV prophylaxis with antiviral medications, such as valganciclovir, is often used to prevent CMV infection in this population.

**Correct Answer:** C.