Question:easy
Child. Sunlight causes eruptions, diagnosed as DNA repair defect. Which defect can it be?
Child. Sunlight causes eruptions, diagnosed as DNA repair defect. Which defect can it be?
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Sunlight → UV → thymine dimers → these bulky lesions are cleared by nucleotide excision repair (defect = Xeroderma Pigmentosum).
Updated On: Jun 22, 2026
- Nucleotide excision
- Base excision repair
- Mismatch repair defect
- Recombination defect
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The Correct Option is A
Solution and Explanation
Pattern-match the lesion to the pathway. The key is to translate the clinical clue into the molecular lesion. "Sunlight causes eruptions" means ultraviolet exposure, and UV light's signature DNA damage is the pyrimidine dimer - a bulky, helix-distorting lesion formed between adjacent thymines.
Each DNA repair system handles a specific type of damage:
• Bulky, helix-distorting lesions (UV dimers, chemical adducts) → repaired by Nucleotide Excision Repair.
• Small single-base alterations (oxidised/deaminated bases) → Base Excision Repair.
• Replication mismatches and indel loops → Mismatch Repair.
• Double-strand breaks → Homologous Recombination / Non-homologous end joining.
Because the patient's damage is UV-induced pyrimidine dimers, the failed pathway must be Nucleotide Excision Repair. The disease itself is Xeroderma Pigmentosum, where inability to excise UV dimers leads to extreme sun sensitivity and early skin cancers.
Therefore the correct defect is Nucleotide Excision (Option A).
Each DNA repair system handles a specific type of damage:
• Bulky, helix-distorting lesions (UV dimers, chemical adducts) → repaired by Nucleotide Excision Repair.
• Small single-base alterations (oxidised/deaminated bases) → Base Excision Repair.
• Replication mismatches and indel loops → Mismatch Repair.
• Double-strand breaks → Homologous Recombination / Non-homologous end joining.
Because the patient's damage is UV-induced pyrimidine dimers, the failed pathway must be Nucleotide Excision Repair. The disease itself is Xeroderma Pigmentosum, where inability to excise UV dimers leads to extreme sun sensitivity and early skin cancers.
Therefore the correct defect is Nucleotide Excision (Option A).
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