What are six causes of thickened duodenal folds?

1. Duodenitis (e.g., as a result of peptic ulcer disease or Zollinger-Ellison syndrome)
2. Infection (Giardia lamblia, Strongyloides stercoralis, Mycobacterium tuberculosis)
3. Paraduodenal inflammation (e.g., pancreatitis, cholecystitis)
4. Infiltration (e.g., Whipple's disease)
5. Vascular disorders (e.g., varices, hemorrhage)
6. Neoplasia (e.g., lymphoma)