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: Biochemistry
Pyrimidine dimers are seen in: 
Select and explain. 
A. Xeroderma pigmentosum, B. Alkylating agents, C. X-rays, D. Gamma rays
A: Xeroderma pigmentosum
Answer: A: Xeroderma pigmentosum 
Explanation: DNA damaging agent that results in pyrimidine dimer is U-V light chemicals:

DNA repair mechanism that repair pyrimidine dimer is Nucleotide Excision repair.
Defective Nucleotide excision repair mechanism causes Xeroderma pigmentosum, Cockayne Syndrome, Trichothiodystrophy.
So, pyrimidine dimers are seen in disorders caused by defective nucleotide excision repair.

Reference- Harper 30th edition Pg - 390