**Core Concept**
Gancyclovir is an antiviral medication that acts as a nucleoside analog, specifically targeting herpesvirus infections. Its mechanism of action involves inhibiting viral DNA synthesis, which is essential for the replication of herpesviruses.

**Why the Correct Answer is Right**
Gancyclovir is more effective than acyclovir against cytomegalovirus (CMV) infections. This is because gancyclovir has a higher affinity for the viral DNA polymerase of CMV, which is the enzyme responsible for viral DNA replication. As a result, gancyclovir is able to inhibit CMV replication more effectively than acyclovir. Additionally, gancyclovir has a longer half-life and higher bioavailability than acyclovir, allowing for more sustained and effective antiviral activity.

**Why Each Wrong Option is Incorrect**
* **Option A:** Acyclovir is more effective than gancyclovir against herpes simplex virus (HSV) and varicella-zoster virus (VZV), not CMV. While acyclovir is effective against these viruses, gancyclovir's higher efficacy against CMV outweighs its lower efficacy against HSV and VZV.
* **Option B:** Gancyclovir is not more effective than acyclovir against Epstein-Barr virus (EBV). Both medications have limited efficacy against EBV, and gancyclovir's higher efficacy against CMV does not translate to EBV.
* **Option C:** Gancyclovir is not more effective than acyclovir against human herpesvirus 6 (HHV-6). While gancyclovir has some activity against HHV-6, its efficacy is not significantly greater than acyclovir's.

**Clinical Pearl / High-Yield Fact**
When treating CMV infections, gancyclovir should be used as first-line therapy due to its higher efficacy and longer half-life compared to acyclovir. However, acyclovir may still be used as an adjunctive therapy or in cases where gancyclovir is contraindicated.

**Correct Answer:** B. CMV