<prompt> A 10 year old child experiences recurrent skin lesions on her face and upper extremities that rapidly progress to cancer. Xeroderma pigmentosa is considered a possible differential. Which of the following is defective in that caseTopoisomerase3 5 ExonucleaseEndonucleaseHelicase  <response>: UV rays damage DNA by forming thymidine dimers. This is repaired by the body by nucleotide excision repair. Main enzyme of this mechanism is UV-specific endonuclease. 