In a patient with a positive HLA-B*5701 test presenting with fever and rash after starting antiretroviral therapy, which drug is most likely responsible for the reaction?

In this scenario, the presence of a positive HLA-B5701 test is critical because it indicates that the patient has a genetic predisposition to hypersensitivity reactions specifically to abacavir, which is an antiretroviral drug used in the treatment of HIV. The HLA-B5701 allele is associated with an increased risk of hypersensitivity, which can manifest as fever, rash, gastrointestinal symptoms, and other systemic reactions shortly after the initiation of therapy.

When a patient with this genetic marker is exposed to abacavir, the immune system may react adversely, leading to the symptoms described. This hypersensitivity reaction can be serious and sometimes life-threatening, making it essential to avoid the drug in individuals who test positive for this genetic marker.

Other options listed, such as zidovudine, efavirenz, and darunavir, are not associated with the HLA-B5701 sensitivity allele and do not typically cause hypersensitivity reactions in the same manner as abacavir. Therefore, given the context of the question and the implications of the HLA-B5701 test result, abacavir is indeed the drug most likely responsible for the adverse reaction in this patient.