Which drug is used exclusively in combo therapies among those listed?

Bictegravir is utilized exclusively in combination therapies, specifically as part of a fixed-dose combination tablet used to treat HIV. This drug is combined with other antiretroviral medications to enhance efficacy and improve compliance by reducing the number of pills the patient needs to take. The strategy of using bictegravir in combination is particularly beneficial for managing HIV, as it allows for a powerful and well-tolerated regimen while minimizing the potential for resistance and side effects.

In contrast, the other options, while also effective in combination therapies, can be used independently or in various combinations with different regimens. Raltegravir, cabotegravir, and dolutegravir can all be prescribed alone or with specific partners based on clinical needs, whereas bictegravir is specifically formulated to be used in conjunction with other agents, thus cementing its role as a drug used exclusively in combination therapies.