B) hereditary defects of lipid metabolism

Tay-Sachs disease or Infantile Amaurotic Idiocy. The child having this disease is born normal, but a few months later, due to an error in lipid metabolism, his brain and spinal cord are damaged. This results in mental retardation and paralysis. The victim dies in 3 or 4 years. These is no treatment for this disease.

Tay-Sachs disease is caused by homozygosity for a rare recessive mutation that maps to chromosome 15.

Lysosomes are membrane-bound organelles that contain 40 or more different digestive enzymes.

These enzymes catalyze the breakdown of nucleic acids, proteins, polysaccharides, and lipids. A number of human diseases are caused by mutations in gene that code for lysosomal enzymes. These diseases called lysosomal-storage diseases, are generally caused by recessive mutations.

In Tay-Sachs infants the enzyme is nonfunctional, so the unprocessed ganglioside accumulates in the brain cells. This accumulation causes cerebral degeneration and death, usually by age three.

Early diagnosis is also made possible by the presence on the retina of a cherry-colored spot surrounded by a white halo. Rapid neurological degeneration involves generalized paralysis, blindness, a progressive loss of hearing, and serious feeding problems.

By two years of age the infants are essentially immobile, and death occurs at about three to four years of age, often from respiratory infections.