A 27-year-old man develops malaise, fatigue, and loss of appetite three weeks after a meal at the Trucker's Cafe. He notes passing dark urine. On physical examination, he has mild scleral icterus and right upper quadrant tenderness. Laboratory studies show serum AST of 62 U/L and ALT of 58 U/L. The total bilirubin concentration is 3.9 mg/dL, and the direct bilirubin concentration is 2.8 mg/dL. His symptoms abate over the next 3 weeks. On returning to the cafe, he finds that the city's health department has closed it. Which of the following serologic test results is most likely to be positive in this patient?