(e) Obturator hernia: This is the most likely diagnosis because obturator hernias are more common in elderly females and can cause small bowel obstruction without any history of previous surgery or external hernia. They are often not palpable and can be difficult to diagnose.

(b) Volvulus: This is a possible cause of small bowel obstruction, especially in the elderly, as it can occur without any previous surgery or external hernia. Volvulus refers to the twisting of a portion of the intestine, which can cut off blood supply and lead to obstruction.
(c) Gallstone ileus: Although less common, gallstone ileus is a possible cause of small bowel obstruction in elderly females. It occurs when a gallstone enters the bowel through a fistula and obstructs the intestine.

(a) Small bowel neoplasm: Although not as common as other causes, a small bowel neoplasm can cause obstruction and must be considered to avoid missing a potentially treatable malignancy.
(d) Abdominal abscess: An abdominal abscess can cause small bowel obstruction by compressing or adhering to the bowel. It's crucial to identify and treat the underlying infection to prevent severe complications.

Other rare causes of small bowel obstruction in this demographic might include internal hernias not associated with previous surgery, intussusception, or other unusual conditions. However, these are less likely and would be considered after ruling out the more common causes.