A child develops blisters on exposure to sunlight’ Irregular dark spots on the skin are also found’ He is very likely to have a defect in which of the following mechanisms?
**Core Concept**
The child's symptoms suggest a condition where exposure to sunlight triggers an abnormal skin response, leading to blister formation and dark spots. This indicates a defect in the body's ability to repair DNA damage caused by ultraviolet (UV) radiation.
**Why the Correct Answer is Right**
The correct answer is related to the repair of UV-induced DNA damage. UV radiation can cause cyclobutane pyrimidine dimers (CPDs) and 6-4 photoproducts, which are types of DNA lesions. The body has mechanisms to repair these lesions, including nucleotide excision repair (NER) and base excision repair (BER). A defect in the NER pathway, specifically in the gene encoding for the protein xeroderma pigmentosum complementation group A (XPA), would lead to an inability to repair these lesions, resulting in blistering and dark spots upon sun exposure. This is a hallmark of xeroderma pigmentosum, a rare genetic disorder.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because defects in DNA replication and recombination mechanisms are not directly related to the formation of blisters and dark spots upon sun exposure.
**Option B:** This option is incorrect because defects in the mismatch repair pathway are not directly related to the repair of UV-induced DNA damage.
**Option C:** This option is incorrect because defects in the mismatch repair pathway are not directly related to the repair of UV-induced DNA damage.
**Option D:** This option is incorrect because defects in the homologous recombination repair pathway are not directly related to the repair of UV-induced DNA damage.
**Clinical Pearl / High-Yield Fact**
Xeroderma pigmentosum is a rare genetic disorder characterized by an inability to repair UV-induced DNA damage, leading to an increased risk of skin cancers and other complications.
**Correct Answer: D. Nucleotide Excision Repair (NER) pathway**