A 1-year-old female infant has failure to thrive, poor neurologic development, and poor motor function. Physical examination shows a "cherry red" spot on the macula of the retina. The infant's muscle tone is poor. Both parents and a brother and sister are healthy, with no apparent abnormalities. One brother with a similar condition died at the age of 18 months. This genetic disorder most likely resulted from a mutation involving a gene encoding for which of the following?

The findings listed suggest a severe inherited neurologic disease, and the pattern of inheritance (e.g., normal parents, an affected sibling) is consistent with an autosomal recessive disorder. This inheritance pattern and the cherry red spot in the retina are characteristic of Tay-Sachs disease, caused by mutations in the gene that encodes a lysosomal enzyme hexosaminidase A. Mitochondrial genes have a maternal pattern of transmission. Mutations in genes affecting receptor proteins and structural proteins typically give rise to an autosomal dominant pattern of inheritance.