A 28-year-old woman with chronic hepatitis C virus (HCV) genotype 1 infection presents to the clinic complaining of progressive fatigue and dyspnea on exertion for the past week. She is on her eighth week of antiviral therapy with a combination regimen consisting of a guanosine analog, an NS5A inhibitor, an NS3/4A protease inhibitor, and pegylated interferon alfa. Physical examination reveals mild scleral icterus but no hepatosplenomegaly. Laboratory studies show hemoglobin 9.2 g/dL (baseline 13.1 g/dL), elevated lactate dehydrogenase, indirect hyperbilirubinemia, increased reticulocyte count, and decreased haptoglobin. Which of the following medications in her regimen is most likely responsible for her hematologic findings?