A 54-year-old forestry worker presents with a 3-year history of migratory large-joint arthralgia followed by chronic diarrhea (5–7 loose fatty stools daily), 12 kg weight loss, abdominal bloating, and progressive fatigue. He also has low-grade fever, generalized lymphadenopathy, and subtle cognitive slowing. Investigations reveal microcytic anaemia, hypoalbuminemia, positive faecal fat, and negative celiac serology. Upper GI endoscopy is performed. The structure marked **A** in the diagram—the duodenal yellow-white plaques—is characteristic of which condition?