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 dimmers
A: Formation of thymidine dimmers
Answer: A: Formation of thymidine dimmers 
Explanation: In xerodenna pigmentosa; nucleotide excision repair of damaged DNA is defective due to formation of TT dimers.