**Core Concept**
Rejection and infections are major complications in renal transplantation. The immune system's response to a transplanted kidney can be suppressed with immunosuppressive drugs, but this also increases the risk of infections. One month after transplantation, the risk of certain infections increases due to a combination of factors, including immunosuppression and the reactivation of latent viruses.

**Why the Correct Answer is Right**
CMV (Cytomegalovirus) infection is a common complication in renal transplant recipients, particularly between 1-3 months post-transplantation. This is because CMV is a herpesvirus that remains latent in the body after initial infection and can reactivate in immunocompromised individuals. The reactivation of CMV can lead to a range of symptoms, including fever, fatigue, and organ dysfunction. CMV infection is often seen in transplant recipients who are receiving immunosuppressive therapy.

**Why Each Wrong Option is Incorrect**
* **Option A:** While BK virus nephropathy is a significant concern in renal transplant recipients, it typically presents earlier (around 3-6 months post-transplantation) and is associated with a different set of clinical symptoms.
* **Option B:** Acute rejection is a common complication in renal transplant recipients, but it typically presents earlier (within 1-3 weeks post-transplantation) and is associated with a different set of clinical symptoms, including graft dysfunction and elevated creatinine levels.
* **Option D:** Herpes simplex virus (HSV) infection is a possible complication in renal transplant recipients, but it is less common than CMV and typically presents with oral or genital lesions.

**Clinical Pearl / High-Yield Fact**
CMV infection is often prevented with antiviral prophylaxis in renal transplant recipients. However, this can be discontinued after 3-6 months post-transplantation, which increases the risk of CMV reactivation.

**Correct Answer: C. CMV (Cytomegalovirus)**