A 40-year-old male presents with fever for 3 days accompanied by abdominal pain and diarrhea, passing stool over 20 times daily with small volume, containing mucus and pus/blood. Stool microscopy shows red blood cells (++), white blood cells (++), and mucus (++). A diagnosis of acute bacillary dysentery is made. Which segment of the intestine is primarily affected by the pathological changes in this disease?