Schistosomal Hepatic Fibrosis

A 45-year-old man from rural Sudan presents with massive splenomegaly, thrombocytopenia (platelets 60,000/μL), and a history of two episodes of hematemesis from esophageal varices in the past 18 months. Liver synthetic function is preserved (albumin 3.8 g/dL, INR 1.1, total bilirubin 0.9 mg/dL). Stool microscopy reveals lateral-spined eggs. Abdominal ultrasound shows echogenic periportal thickening with a "turtle-back" pattern and patent portal vein. The pathologic process marked **B** in the diagram is responsible for his portal hypertension. Which of the following BEST characterizes the mechanism by which this lesion causes portal hypertension?