<prompt> The commonest site of obstruction in Gallstone ileus is -Proximal ileumDistal ileumIleocecal junctionTransverse colon  <response>: Although Harrison writes the most common site to be the ileocecal valve; Sabiston, Bailey Maingots write distal ileum to be the most common site. The site of obstruction is usually the terminal ileum, just proximal to the ileocecal valve. This corresponds to the segment of the intestine with the smallest diameter. Bailey Love write, classically there is impaction about 60 cm proximal to the ileocecal valve. Also, know the commonest site for gallstone impaction in CBD - Ampulla of Vater. Ref : Maingots 10e, p 1736 