A 5-year-old child presents with extreme photosensitivity, severe sunburns with minimal sun exposure, freckling, and early development of basal cell carcinoma. This autosomal recessive condition is caused by a defect in:

Answer and explanation

Correct answer: B. Nucleotide excision repair (Xeroderma pigmentosum)

  1. Step 1: Recognize the UV-induced DNA damage mechanism: ultraviolet radiation induces covalent cross-linking between adjacent pyrimidine bases, forming cyclobutane pyrimidine (thymine) dimers that distort the DNA helical backbone.
  2. Step 2: Bulky helix-distorting DNA lesions are repaired by nucleotide excision repair (NER), where specific endonucleases cleave the damaged oligonucleotide strand. In Xeroderma pigmentosum, mutations in NER genes prevent dimer excision, resulting in extreme photosensitivity and high skin cancer incidence.
Answer
Nucleotide excision repair (Xeroderma pigmentosum).
Why not the others: Base excision repair removes single damaged bases (such as deaminated cytosine to uracil) via DNA glycosylases and AP endonucleases; Mismatch repair (Lynch syndrome) corrects single-base mismatches and small insertion/deletion loops during DNA replication; Non-homologous end joining (Ataxia telangiectasia) repairs double-strand DNA breaks caused by ionizing radiation.

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