This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: ENT
[{'content': 'Most accepted theory for the formation of secondary cholesteatoma: \nSelect and explain. \nA. Congenital, B. Squamous metaplasia, C. Ingrowth of squamous epithelium, D. Retraction pocket', 'role': 'user'}, {'content': 'Answer: D \nExplanation: According to this theory, chronic negative middle ear pressure (which occurs due to poor Eustachian tube function and chronic inflammation of the middle ear) leads to retractions of the structurally weakest area of the tympanic membrane, the pars flaccida. Once the retractions form, the normal migratory pattern of the squamous epithelium is disrupted, resulting in the accumulation of keratin debris in the cholesteatoma sac..', 'role': 'assistant'}]