**Core Concept**
Syncytium formation is a characteristic cytopathic effect (CPE) produced by certain viruses, particularly those belonging to the Togaviridae and Coronaviridae families. This CPE is characterized by the fusion of infected cells, resulting in the formation of a multinucleated giant cell or syncytium.

**Why the Correct Answer is Right**
Syncytium formation is a result of the fusion of infected cells, which is mediated by the viral glycoprotein responsible for cell-to-cell transmission. In the case of Togaviridae and Coronaviridae, this glycoprotein is encoded by the viral genome and is responsible for the fusion of host cells. The formation of syncytia is a hallmark of infection by these viruses and can be observed in various cell cultures.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because syncytium formation is not typically associated with viruses belonging to the Picornaviridae family, such as poliovirus or rhinovirus.
**Option B:** Incorrect because syncytium formation is not a characteristic CPE of viruses belonging to the Herpesviridae family, such as herpes simplex virus.
**Option C:** Incorrect because syncytium formation is not a typical CPE of viruses belonging to the Adenoviridae family, such as adenovirus.

**Clinical Pearl / High-Yield Fact**
It's essential to note that syncytium formation can be a diagnostic clue for certain viral infections, particularly those caused by Togaviridae and Coronaviridae. In clinical settings, observing the formation of multinucleated giant cells in cell cultures can aid in the identification of these viruses.

**Correct Answer: D.**