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 case: (A) Topoisomerase, (B) 3' → 5' Exonuclease, (C) Endonuclease, (D) Helicase
Answer is C. 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.