Tay-sachs disease is a disorder characterized by an accumulation of fatty substances called gangliosides in the brain and spinal cord. one cause of the disease is a mutation in the gene sequence that codes for the enzyme hexosaminidase a. the enzyme is rendered nonfunctional when a single amino acid—glutamine—is incorporated into the enzyme instead of an arginine. in this scenario, what is the cause of tay-sachs disease?

In this case, the cause of tay-sachs disease is a point mutation of the type missense.

it's called a point mutation when a single nucleotide base is changed, inserted or deleted from a DNA or RNA sequence . A subtype of this mutation is a missense mutation and that's when a amino acid is switched by another.
It's possible that a switch in the following codons caused the disease:
glutamine codons: CAA, CAG
Arginine's codons: CGU, CGC, CGA, CGG, AGA, and AGG