Intussusception: This condition, where a part of the intestine telescopes into another, can cause a smooth change in the caliber of the distal ileal loops due to the invagination of one intestinal segment into another. It is a common cause of intestinal obstruction, especially in children but can also occur in adults.

Adhesions: Post-surgical adhesions can cause a smooth change in the caliber of the intestine by creating a gradual narrowing of the intestinal lumen, leading to obstruction. This is a common cause of small bowel obstruction in patients with a history of abdominal surgery.
Hernias: Internal or external hernias can cause obstruction by compressing the intestine at the hernia neck, leading to a smooth transition from normal to collapsed bowel loops.
Tumors: Both benign and malignant tumors can cause obstruction by gradually narrowing the intestinal lumen, although the change in caliber might not always be smooth.

Volvolus: Although volvulus typically presents with a more abrupt change in caliber, it's crucial not to miss this diagnosis due to its high mortality rate if not promptly treated. Volvulus involves the twisting of a portion of the intestine, which can lead to ischemia and necrosis.
Ischemic Stricture: Ischemic injury to the bowel can result in stricture formation, leading to obstruction. The history and clinical presentation might suggest this diagnosis, but it's critical due to its potential for severe complications.
Inflammatory Bowel Disease (IBD): Conditions like Crohn's disease can cause strictures leading to obstruction. While the change in caliber might not always be smooth, missing this diagnosis could lead to inappropriate management and significant morbidity.

Lymphoma: Intestinal lymphoma can cause obstruction by either mass effect or by leading to intussusception. It's less common than other causes but should be considered, especially in patients with systemic symptoms or a history of lymphoma.
Carcinoid Tumors: These neuroendocrine tumors can cause fibrosis and strictures, leading to obstruction. They are less common than other tumors but can have significant implications if not diagnosed and managed properly.
Meckel's Diverticulum: A congenital anomaly that can cause obstruction either by acting as a lead point for intussusception or by causing volvulus. It's a rare cause of obstruction in adults but should be considered in the differential diagnosis, especially in younger patients.