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
Patient present with skin bullae on the sun exposure. The defect is of 
Select and explain. 
A. Sugar changes, B. DNA methylation, C. Thymidine dimers, D. Trinucleotide repeats
C: Thymidine dimers
Answer: C: Thymidine dimers 
Explanation: 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.