The following diseases have a defect in DNA repair mechanism except for
**Core Concept**
The question is testing the student's knowledge of DNA repair mechanisms, specifically the types of genetic disorders that result from defects in these mechanisms. DNA repair is a crucial process that corrects damage to the genetic material, and deficiencies in this process can lead to various genetic disorders.
**Why the Correct Answer is Right**
The correct answer is a disease that does not involve a defect in DNA repair mechanism. DNA repair mechanisms include base excision repair (BER), nucleotide excision repair (NER), mismatch repair (MMR), and homologous recombination repair (HRR). Diseases that involve defects in these mechanisms include xeroderma pigmentosum (NER), hereditary nonpolyposis colorectal cancer (MMR), and ataxia-telangiectasia (HRR). The correct answer is a disease that does not involve a defect in one of these DNA repair mechanisms.
**Why Each Wrong Option is Incorrect**
**Option A:** This disease is caused by a defect in the **BRCA1** gene, which is involved in homologous recombination repair (HRR). Defects in HRR can lead to an increased risk of breast and ovarian cancer.
**Option B:** This disease is caused by a defect in the **MSH2** gene, which is involved in mismatch repair (MMR). Defects in MMR can lead to an increased risk of colorectal cancer and other types of cancer.
**Option C:** This disease is caused by a defect in the **CFTR** gene, which is involved in cystic fibrosis, a genetic disorder that affects the lungs and other organs. Cystic fibrosis is not caused by a defect in DNA repair mechanism.
**Clinical Pearl / High-Yield Fact**
It's essential to remember that defects in DNA repair mechanisms can lead to an increased risk of cancer and other genetic disorders. This knowledge is crucial for identifying patients who may benefit from genetic testing and surveillance.
**Correct Answer:** C. Cystic fibrosis