A 47-year-old man with longstanding HIV infection and prior treatment with zidovudine, lamivudine, efavirenz, lopinavir/ritonavir, and raltegravir presents for follow-up. Recent genotypic testing reveals high-level resistance to reverse transcriptase, protease, and integrase inhibitors. His current regimen comprises two nucleoside reverse transcriptase inhibitors plus an agent administered subcutaneously that binds gp41 to prevent viral fusion. Two weeks after initiation, he develops multiple painful erythematous nodules at injection sites. Which agent was most likely added to his regimen?