Correct answer:

Acute appendicitis usually occurs when the appendix is obstructed by a faecolith or foreign body in the lumen, by a fibrous stricture in its wall from previous inflammation, or by enlargement of lymphoid follicles in its wall secondary to inflammation of its mucosa; rarely it is associated with an obstructing carcinoid tumour near its base. Occasionally, acute appendicitis occurs proximal to an obstructing lesion (usually carcinoma) in the caecum or ascending colon.