Final answer:
Xeroderma pigmentosum (XP) is caused by a defective nucleotide excision repair mechanism, preventing proper repair of thymine dimers caused by UV exposure and increasing the risk of skin cancer.
Step-by-step explanation:
Xeroderma pigmentosum (XP) results from a defective nucleotide excision repair mechanism. When individuals with XP are exposed to UV light, pyrimidine dimers, most notably thymine dimers, are formed. These individuals cannot effectively repair the damage due to a defect in the nucleotide excision repair enzymes. As a result, the thymine dimers remain in the DNA, distorting the DNA double helix structure and leading to complications during DNA replication, which significantly increases the risk of contracting skin cancer.