Question

what causes huntingtons disease?
a mutation causing an inability to break down fatty tissue around the brain
an altered shape of hemoglobin where it folds incorrectly
a mutation of a repeated codon cag that repeats 40 or more times

Explanation:

Huntington's disease is a genetic disorder. It is caused by a mutation in the HTT gene. This gene has a CAG trinucleotide repeat. When the number of CAG repeats reaches 40 or more, it leads to the production of a toxic protein. This toxic protein causes damage to nerve cells in the brain, resulting in the symptoms of Huntington's disease.
The first option describes a condition related to lipid metabolism disorders (like some leukodystrophies), not Huntington's. The second option is related to sickle - cell anemia (due to abnormal hemoglobin shape), not Huntington's.

Answer:

a mutation of a repeated codon CAG that repeats 40 or more times