A 25-year-old woman presented to the clinic 1 week ago with fever, abdominal pain, and diarrhea (20+ episodes/day of bloody mucoid stools, scant volume, with tenesmus). She was treated for acute bacillary dysentery with oral antibiotics for 5 days with symptom improvement. Yesterday, after eating spoiled fruit, she developed recurrent abdominal pain, diarrhea (10+ episodes/day), and tenesmus. Which test is most definitive for diagnosing bacillary dysentery?