This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Genetics, Subject name: Pathology
[{'content': 'Patient present with skin bullae on sun exposure. There is a defect in which of the following? \nSelect and explain. \nA. Thymidine dimers, B. Trinucleotide repeats, C. Sugar changes, D. DNA methylation', 'role': 'user'}, {'content': 'Answer: A \nExplanation: Xeroderma pigmentosum Autosomal recessive disorder Defective DNA repair. Increased risk for skin cancers following exposure to the UV light UV radiation- Cross-linking of pyrimidine residues,-Preventing normal DNA replication. Such DNA damage is repaired by the nucleotide excision repair system. Deficiency of UV specific endonuclease (commonest), DNA polymerase and DNA ligase', 'role': 'assistant'}]