Following a dog bite from a "probably rabid" animal, what should be administered to an unvaccinated adult?

For an unvaccinated adult who has been bitten by a "probably rabid" animal, the correct course of action is to administer both human rabies immune globulin and human diploid cell rabies vaccine. This combined approach provides immediate and long-lasting protection against rabies, a viral infection that has a very high fatality rate once clinical symptoms appear.

The human rabies immune globulin (HRIG) is critical because it provides passive immunity. This means it supplies immediate antibodies against the rabies virus to the person who has been exposed, which can effectively neutralize the virus at the site of the bite. The human diploid cell rabies vaccine (HDCV) is then administered to induce active immunity. It helps the body produce its own antibodies against the virus over time, providing long-term protection.

In the case of unvaccinated individuals, not using HRIG along with the vaccine (as seen in the other options) would significantly increase the risk of developing rabies, which is often fatal. Therefore, the combination treatment is essential to ensure the best chance of preventing the disease following a potentially rabid exposure.