Which genetic disorder is caused by a mutation in hemoglobin that leads to sickle-shaped red blood cells?

Sickle-cell disease is caused by a specific mutation in the hemoglobin gene, which leads to the production of abnormal hemoglobin known as hemoglobin S. This mutation prompts the red blood cells to take on a characteristic sickle or crescent shape, particularly under low-oxygen conditions. These sickle-shaped cells can lead to blockages in blood flow, reducing oxygen delivery to tissues and causing various complications such as pain crises, increased risk of infection, and organ damage.

In contrast, the other genetic disorders mentioned have different underlying causes. Marfan syndrome is a connective tissue disorder caused by mutations in the FBN1 gene, affecting the body's connective tissues. Tay-Sachs disease is a neurodegenerative condition caused by a deficiency in the enzyme hexosaminidase A due to mutations in the HEXA gene, leading to the accumulation of GM2 gangliosides in nerve cells. Huntington's disease is caused by a mutation in the HTT gene, leading to neurodegeneration over time, primarily characterized by movement disorders and cognitive decline. Thus, the unique mutation in hemoglobin that causes sickle-shaped red blood cells distinctly identifies sickle-cell disease as the correct answer.