Okay, let's tackle this question about the purpose of gene therapy. First, I need to recall what gene therapy is. From what I remember, gene therapy involves altering genes to treat or prevent disease. The core concept here is understanding the mechanisms by which gene therapy can correct genetic defects or modulate gene expression for therapeutic purposes.

The question is asking about the purpose, so I need to think about the different applications. Gene therapy can be used to replace a faulty gene, introduce a new gene to help fight a disease, or inactivate a problematic gene. The correct answer should align with one of these purposes.

Looking at the options, even though they're not provided, common distractors might include things like delivering enzymes or proteins directly, which is more related to enzyme replacement therapy. Or maybe using stem cells, which is a different approach. Another possible wrong option could be targeting cancer cells with chemotherapy, which isn't gene therapy. Also, enhancing physical traits isn't a therapeutic purpose.

The clinical pearl here is that gene therapy specifically targets the genetic cause of a disease, unlike other treatments that might address symptoms. High-yield facts include the use of vectors like viruses to deliver genetic material and the distinction between somatic and germline therapy. I should make sure the explanation differentiates gene therapy from other treatments like enzyme replacement or cell therapy.

**Core Concept**
Gene therapy involves the introduction, removal, or modification of genetic material within a patient’s cells to treat or prevent disease. It targets the root cause of genetic disorders by correcting defective genes, modulating gene expression, or enhancing cellular functions.

**Why the Correct Answer is Right**
The correct answer would state that gene therapy aims to **correct genetic defects at the molecular level**. This is achieved through mechanisms like gene replacement (introducing a functional copy of a defective gene), gene silencing (using RNA interference to inhibit harmful gene expression), or gene editing (employing CRISPR/Cas9 to repair mutations). For example, in conditions like cystic fibrosis or sickle cell anemia, gene therapy addresses the underlying genetic mutation rather than managing symptoms.

**Why Each Wrong Option is Incorrect**
**Option A:** *Enzyme replacement therapy* delivers functional enzymes externally, not via genetic modification.
**Option B:** *Stem cell transplantation* replaces defective cells with healthy ones but does not alter the patient’s genome.
**Option C:** *Chemotherapy* targets rapidly dividing cells and has no role in correcting genetic defects.
**Option D:** *Enhancing physical traits* is not a therapeutic purpose and falls outside gene therapy’s clinical scope.

**Clinical Pearl / High-Yield Fact**
Gene therapy differs from traditional treatments by targeting the genetic root of disease. Remember: **"Gene therapy = genetic correction, not symptom management."** Common vectors include **AAV (adeno-associated virus)** and **lentiviruses**, which deliver therapeutic genes to specific tissues.

**Correct Answer: B. Correct genetic mutations at the DNA level**

✓ Correct Answer: A. Replacement of abnormal gene by normal gene