**Core Concept**
Gene therapy is a medical treatment that uses genes to prevent or treat diseases. It involves modifying or replacing a gene to restore normal function, or introducing a new gene to prevent disease. Gene therapy can be used to treat various conditions, including inherited disorders, cancer, and viral infections.

**Why the Correct Answer is Right**
Gene therapy works by introducing healthy copies of a gene into cells to replace faulty or missing genes. This is typically done using a vector, such as a virus, which delivers the gene to the target cells. The gene is then expressed, producing a protein that helps to restore normal function or prevent disease. Gene therapy can be used to treat conditions such as sickle cell anemia, cystic fibrosis, and muscular dystrophy.

**Why Each Wrong Option is Incorrect**
**Option A:** Gene therapy is not typically used for bacterial infections, as antibiotics are usually the treatment of choice. Gene therapy may be used to prevent or treat bacterial infections in the future, but it is not currently a standard treatment.
**Option B:** Gene therapy is not used to treat viral infections by killing the virus, but rather by modifying the host cells to prevent the virus from replicating.
**Option C:** Gene therapy is not used to treat cancer by killing cancer cells, but rather by modifying the host cells to prevent cancer cells from growing or by introducing genes that help to kill cancer cells.

**Clinical Pearl / High-Yield Fact**
Gene therapy is a rapidly evolving field, and new treatments are being developed to treat a wide range of diseases. It is essential to stay up-to-date with the latest research and developments in this field to provide optimal care for patients.

**Correct Answer: D. Leber Congenital Amaurosis