This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Metabolism of nucleic acids, Subject name: Biochemistry
[{'content': 'Patient present with skin bullae on the sun exposure. The defect is of \nSelect and explain. \nA. Sugar changes, B. DNA methylation, C. Thymidine dimers, D. Trinucleotide repeats', 'role': 'user'}, {'content': 'Answer: C \nExplanation: A pair of abnormally chemically bonded adjacent thymine bases in DNA, resulting from damage by ultra-violet irradiation. The cellular processes that repair this lesion often make errors that create mutations.', 'role': 'assistant'}]