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: Medicine
Friedreich's ataxia is caused due to triplet repeats of: 
Select and explain. 
A. AGG, B. GAA, C. UAA, D. AUG
B: GAA
Answer: B: GAA 
Explanation: The classic form of Friedreich's ataxia has been mapped to 9q13-q21.1, and the mutant gene, frataxin, contains expanded GAA triplet repeats in the first intron. There is homozygosity for expanded GAA repeats in more than 95% of patients. Normal persons have 7-22 GAA repeats, and patients have 200-900 GAA repeats. A more varied clinical syndrome occurs in compound heterozygotes who have one copy of the GAA expansion and the other copy a point mutation in the frataxin gene. When the point mutation is located in the region of the gene that encodes the amino-terminal half of frataxin, the phenotype is milder, often consisting of a spastic gait, retained or exaggerated reflexes, no dysahria, and mild or absent ataxia. Ref: Harrison's principle of internal medicine 17th edition.