A 27-year-old man with a history of intravenous drug use is known to have been infected with the hepatitis B virus for the past 6 years and has not been ill. He is seen in the emergency department because he has had nausea, vomiting, and passage of dark-colored urine for the past week. Physical examination shows scleral icterus and mild jaundice. Neurologic examination shows a confused, somnolent man-oriented only to person. He exhibits asterixis. Laboratory studies show total protein, 5 g/dL; albumin, 2.7 g/dL; AST, 2342 U/L; ALT, 2150 U/L; alkaline phosphatase, 233 U/L; total bilirubin, 8.3 mg/dL; and direct bilirubin, 4.5 mg/dL. Superinfection with which of the following viruses has most likely occurred in this man?