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
The primary defect in Xeroderma pigmentosa is : 
Select and explain. 
A. Formation of thymidine dimmers, B. Poly ADP ribose polymerase is defective, C. Exonuclease is defective, D. Formation of adenine dimers
A: Formation of thymidine dimmers
Answer: A: Formation of thymidine dimmers 
Explanation: A i.e. Formation of thymidine dimmers