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: Pathology
[{'content': 'In Huntington chorea the causative mutation in the protein huntingtin is a \nSelect and explain. \nA. Trinucleotide repeat expansion, B. Gene deletion, C. Frameshift mutation, D. Point mutation', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Expansion of trinucleotide repeats is an important genetic cause of human disease, particularly neurodegenerative disorders. There are three key mechanisms by which unstable repeats cause diseases:\n• Loss of function of the affected gene occurs in fragile X syndrome. In such cases the repeats are generally in noncoding part of the gene.\n• A toxic gain of function by alterations of protein structure as in Huntington disease and spinocerebellar ataxias. In such cases the expansions occur in the coding regions of the genes.\n• A toxic gain of function mediated by mRNA as is seen in fragile X tremor-ataxia syndrome. In this condition, the non coding parts of the gene are affected.', 'role': 'assistant'}]