The primary defect in Xeroderrna pigmentosa is
**Core Concept**
Xeroderma pigmentosum (XP) is a rare genetic disorder characterized by an inability to repair DNA damage caused by ultraviolet light from the sun or other sources. This leads to an accumulation of mutations in the skin, causing a range of symptoms including photosensitivity, skin cancer, and premature aging.
**Why the Correct Answer is Right**
The primary defect in Xeroderma pigmentosum is a deficiency in nucleotide excision repair (NER), a DNA repair mechanism that removes UV-induced damage from DNA. This deficiency is due to mutations in genes involved in the NER pathway, such as **XPA**, **XPC**, **XPD**, **XPF**, **XPG**, **ERCC1**, and **ERCC2**. The inability to repair UV-induced DNA damage leads to an accumulation of mutations, which in turn causes the symptoms associated with XP.
**Why Each Wrong Option is Incorrect**
**Option A:** This option is not a known primary defect in Xeroderma pigmentosum.
**Option B:** This option is incorrect as it is not directly related to the primary defect in XP.
**Option D:** This option is incorrect as it is not a known primary defect in Xeroderma pigmentosum.
**Clinical Pearl / High-Yield Fact**
XP is an autosomal recessive disorder, meaning that individuals must inherit two defective copies of the gene (one from each parent) to develop the condition. This can be a challenge in diagnosis, as carriers may not exhibit symptoms themselves but can pass the defective gene to their offspring.
**Correct Answer:** C. Deficiency in nucleotide excision repair (NER).