A child with a history of severe sunburns and multiple skin cancers presents with extreme photosensitivity. This condition is most likely due to a defect in which DNA repair mechanism?

The correct answer to the question "A child with a history of severe sunburns and multiple skin cancers presents with extreme photosensitivity. This condition is most likely due to a defect in which DNA repair mechanism?" is Option C: Nucleotide excision repair.

The other options — A) Mismatch repair, B) Base excision repair, D) Non-homologous end joining — are incorrect for this USMLE Exam Prep question. As explained above: Xeroderma Pigmentosum is caused by a defect in nucleotide excision repair (NER), which is responsible for removing pyrimidine dimers formed by UV radiation, leading to increased risk of skin cancer.