Which process is primarily affected by inhibitors of endosomal acidification?

The correct choice indicates that viral membrane fusion with host cells is primarily affected by inhibitors of endosomal acidification. This process is crucial for many viruses that rely on the acidic environment of endosomes to facilitate fusion between the viral envelope and the host cell membrane. When a virus enters a host cell via endocytosis, it is transported to an endosome where the acidification typically triggers conformational changes in viral proteins, enabling the virus to merge its membrane with that of the host cell.

If inhibitors block this acidification, the necessary conditions for membrane fusion are disrupted, preventing the virus from entering the cytosol and accessing the cellular machinery needed for subsequent stages of its life cycle. Thus, the fusion step, essential for viral entry, is significantly impaired under these conditions.

The other processes—transcription of viral genes, translation of viral proteins, and replication of viral RNA—occur later in the viral life cycle and are not directly influenced by the endosomal pH; they depend more on events that occur after the virus has successfully fused with the host cell membrane and released its genetic material into the cytosol. Hence, the primary impact of inhibiting endosomal acidification is most clearly seen in the fusion process that allows viral entry.