This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: General pathology, Subject name: Pathology
In Xeroderma Pigmentosum, defect is in? 
Select and explain. 
A. Base pair defect, B. Nucleotide Excision Repair, C. Mismatch repair defect, D. Protein folding
B: Nucleotide Excision Repair
Answer: B: Nucleotide Excision Repair 
Explanation: <p>XERODERMA PIGMENTOSUM Autosomal recessive. Patients with this disorder are more prone to develop cancers of sun exposed skin. The basis for this is defective DNA repair. UV rays in sunlight cause Cross linking of pyrimidine residues, preventing DNA replication. Such DNA damage is repaired by nucleotide excision repair. An inherited loss of any of the proteins involved in excision repair leads to xeroderma pigmentosum. {Reference: Robbins basic pathology 9E pg no.197}</p>