A 57-year-old woman with a 20-year history of celiac disease now presents with weight loss, cramps, and abdominal discomfort. She is adherent to her gluten-free diet, and is not taking any new medications. On examination, her abdomen is soft and there is fullness in the right lower quadrant. A small bowel x-ray reveals an area of narrowing in the distal ileum, and there is thickening and nodularity of the mucosal folds. She undergoes surgical resection of the lesion, and the pathology report confirms a tumor that is more common in patients with celiac disease, Crohn's disease, or depressed immune function such as human immunodeficiency virus (HIV).For the above patient with a small bowel tumor, select the most likely diagnosis.