This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Genetics, Subject name: Pathology
In Huntington chorea the causative mutation in the protein huntingtin is a: 
Select and explain. 
A. Point mutation, B. Gene deletion, C. Frameshift mutation, D. Trinucleotide repeat expansion
D: Trinucleotide repeat expansion
Answer: D: Trinucleotide repeat expansion 
Explanation: Huntington disease (HD) Mutation in the HTT gene which codes for protein called huntingtin which is impoant to neurons. The HTT mutation involves a DNA segment known as a CAG trinucleotide repeat. Made up of a series of three DNA building blocks (cytosine, adenine, and guanine) that appear multiple times in a row leading to the disease.