**Core Concept**
The underlying principle being tested is the knowledge of common infections in immunocompromised patients, specifically those undergoing liver transplantation. **Immunosuppression** increases the risk of various infections. **Opportunistic pathogens** often take advantage of this compromised state.

**Why the Correct Answer is Right**
The most common cause of infection in liver transplant patients is **Cytomegalovirus (CMV)**, due to its prevalence and the immunosuppressed state of these patients. CMV can reactivate from latent infection or be acquired from the donor liver.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because, while bacterial infections are common, they are not the most common cause in this context.
**Option B:** Incorrect as it is less frequently associated with liver transplant infections compared to CMV.
**Option C:** Incorrect because, although fungal infections can occur, they are not the primary concern in liver transplant patients.

**Clinical Pearl / High-Yield Fact**
Remember, CMV infection is a significant concern in transplant patients due to immunosuppression.

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