A 40-year-old male fisherman, residing in a schistosomiasis endemic area with a habit of drinking untreated water, presents with persistent high fever (40°C, remittent type) for 3 weeks, abdominal distension, and mucoid stools 2-3 times daily. Physical exam reveals liver 2 cm below the costal margin and spleen 1 cm below the costal margin. Lab results: WBC count 3.0×10<sup>9</sup>/L (neutrophils 55%, lymphocytes 45%), eosinophil count zero, with accompanying cough. The most likely site of pathology is: