Which medication is indicated to be taken weekly as an antimalarial?

Mefloquine is the correct answer as it is an antimalarial medication specifically indicated for prophylaxis and treatment of malaria. It is typically prescribed to be taken weekly, making it a suitable option for individuals who may be traveling to areas where malaria is prevalent. The weekly dosing regimen is important for maintaining therapeutic levels in the body to effectively prevent malaria infection.

Mefloquine's mechanism of action targets the Plasmodium parasites responsible for malaria, inhibiting their ability to replicate and survive in the bloodstream. This makes it a reliable choice for malaria prevention in high-risk areas.

The other medications listed do not serve as antimalarials. Ritonavir is primarily used as an antiretroviral agent in the treatment of HIV, while Pibrentasvir is an antiviral medication for hepatitis C. Synagis is a monoclonal antibody used primarily for the prevention of respiratory syncytial virus (RSV) infection in infants, not for malaria. Thus, mefloquine uniquely fits the description of a weekly antimalarial medication among the provided options.