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
[{'content': 'The primary defect in Xeroderma pigmentosa is : \nSelect and explain. \nA. Formation of thymidine dimmers, B. Poly ADP ribose polymerase is defective, C. Exonuclease is defective, D. Formation of adenine dimers', 'role': 'user'}, {'content': 'Answer: A \nExplanation: A i.e. Formation of thymidine dimmers', 'role': 'assistant'}]