A study is conducted of patients who are infected with hepatitis virus A, B, C, D, E, or G. The patients are categorized according to the type of virus and are followed over the next 10 years. They receive periodic serologic testing to determine whether they are producing antibodies to the virus with which they were infected. Analysis of the data shows that a cohort of these patients developed antibodies, but subsequently did not clear the virus until treated with pegylated interferon and ribavirin. Which of the following forms of viral hepatitis was most likely to infect this subset of patients?