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': "Carhart's notch in audiometery is seen in - \nSelect and explain. \nA. Ossicular discontinuity, B. Haemotympanum, C. Otomycosis, D. Otosclerosis", 'role': 'user'}, {'content': 'Answer: D \nExplanation: Carharts notch\n\nBone conduction is normal in otosclerosis.\nIn some cases there is a dip in bone conduction curve which is maximum at 2000 Hz / 2 KHZ called as Carharts notch.\nCarharts notch is seen only in bone conduction curve.\nIt disappears after successful stapedectomy/stapedotomy.\nThe reason why it disappears after successful surgery is that when the skull is vibrated by bone—conduction sound, the sound is detected by the cochlea via 3 routes:\n\n– Route (a)—is by direct vibration within the skull.\n– Route (b)—is by vibration of the ossicular chain which is suspended within the skull.\n– Route (c)—is by vibrations emanating into the external auditory canal as sound and being heard by the normal air-conduction route.\n\nIn a conduction type of hearing loss (as in otosclerosis) the latter two routes are deficient but regained by successful reconstruction surgery. Hence bone conduction thresholds improve following surgery.\n\nALSO KNOW\n\nDip in noise induced hearing loss is seen at 4 KHZ.\nIn noise induced hearing loss—Dip is seen in both air and bone conduction curves.\nTrough shaped audiogram is seen in congenital SNHL.\nFlat audiogram with moderate to severes SNHL is characteristic of presbycusis.', 'role': 'assistant'}]