A 31-year-old female presents with persistent high fever, right lower abdominal pain, and diarrhea for 1 week (onset: August 2). Stools occur 3-4 times daily, with mucus and occasional fresh blood. Physical exam: temperature 40°C, heart rate 92/min, lungs clear, 3-4 red rashes on the chest, soft abdomen, mild tenderness in the right lower quadrant, liver 2 cm below the costal margin, spleen palpable 1 cm below the costal margin. Laboratory tests: WBC 4.2×10<sup>9</sup>/L (Neutrophils 60%, Lymphocytes 36%, Monocytes 4%), stool microscopy shows WBC 2-4/HP, RBC 0-2/HP, stool culture negative. The most likely diagnosis is: