This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: None, Subject name: Pathology
Xeroderma pigmentosum is caused due to a defect in: 
Select and explain. 
A. Nucleotide excision repair, B. Base pair, C. Mismatch repair, D. Translocation
A: Nucleotide excision repair
Answer: A: Nucleotide excision repair 
Explanation: ● Defective DNA - repair syndromes are a set of syndrome due to a defect in DNA repair genes and resultant DNA instability.
● These are autosomal recessive in inheritance and include:
Xeroderma pigmentosum
Bloom syndrome
Ataxia telangiectasia.
● Whereas HNPCC is associated with inactivation of DNA mismatch repair gene, resulting in increased susceptibility to colon cancer, and cancers of Endometrium, ovary etc.,
● Example for Base pair defect is Sickle cell anemia; an example for Translocation defect is Philadelphia
chromosome in CML - t (9:22).