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
In which of the following trinucleotide repeat mutation occurs in coding regions of DNA? 
Select and explain. 
A. Myotonic dystrophy, B. Huntington's disease, C. Fragile x syndrome, D. Friedeich ataxia
B: Huntington's disease
Answer: B: Huntington's disease 
Explanation: In Huntington's disease CAG repeat mutation occurs in coding regions of DNA.