A 60-year-old woman presents for evaluation for chronic diarrhoea associated with faecal urgency. No fever, melena, haematochezia, abdominal pain, nausea, vomiting or weight loss. Diarrheal persists even during periods of fasting. Random colonic biopsies demonstrate mononuclear cell infiltration of the lamina propria and a thickened subepithelial collagen band as shown below. What is the most likely diagnosis?