Biochemistry General 0c8aa00f The primary defect in Xeroderrna pigmentosa is A Formation of thymidine dimmers B Poly ADP ribose polymerase is defective C Exonuclease is defective D Formation of adenine dimmers. High-Yield Explanation In xeroderma pigmentos a; nucleotide excision repair of damaged DNA is defective due to formation of TT dimers