This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Miscellaneous, Subject name: Ophthalmology
[{'content': "The toxic agent in 'Tobacco amblyopia' is \nSelect and explain. \nA. Nicotine, B. Cyanide, C. Carbondioxide, D. Dinitrozol", 'role': 'user'}, {'content': 'Answer: B \nExplanation: (Cyanide): Ref: 296-KH (92-P)TOBACCO AMBLYOPIA: -Toxic agent involved cyanide fond in tobacco.Excessive tobacco smoking Decreased cyanide detoxification due to alcoholics dietary deficiency of sulphur rich proteins| |Excessive cyanide in blood | Degeneration of ganglion cells particularly of the macular region | Degeneration of papillo-macular bundle in the nerve | Toxic amblyopia * Bilateral gradually progressive impairment in the central vision* Visual field examination - Bilateral centrocaecal scotomas* Fundus examination is essentially normal or there may be slight temporal pallor of the disc.', 'role': 'assistant'}]