A 25-year-old female presented with fever, abdominal pain, and diarrhea one week ago, passing over 20 stools daily consisting of bloody pus, small in volume, accompanied by tenesmus. She was treated for acute bacillary dysentery with oral antibiotics for 5 days, with symptom improvement. After consuming spoiled fruit yesterday, she experienced recurrent abdominal pain, diarrhea (10+ stools/day), and tenesmus. Currently, the most effective antibiotic for bacillary dysentery is: