**Question:** Cells cultured from patients with this disorder exhibit low activity for the nucleotide excision repair process. This autosomal recessive genetic disease includes marked sensitivity to sunlight (UV light) with subsequent formation of multiple skin cancers and premature death, the disorder is –

A. XP (Xeroderma Pigmentosum)
B. Cockayne Syndrome
C. Celiac Disease
D. Niemann-Pick Disease

**Correct Answer:** A. XP (Xeroderma Pigmentosum)

**Core Concept:** Nucleotide excision repair is an essential DNA repair mechanism that helps remove damaged bases from the DNA strand, preventing mutations and cellular dysfunction. XP is an autosomal recessive genetic disorder affecting nucleotide excision repair, making individuals susceptible to skin cancer formation and premature death.

**Why the Correct Answer is Right:** XP is caused by mutations in genes responsible for nucleotide excision repair proteins. These proteins are essential for detecting and removing damaged nucleotides from the DNA strand. The defective repair mechanism leads to an increased vulnerability to UV light, particularly UVB radiation from sunlight. Upon exposure to UV light, cells with XP exhibit DNA damage, causing mutations and genomic instability. This results in the formation of multiple skin cancers, including basal cell carcinoma, squamous cell carcinoma, and melanoma.

**Why Each Wrong Option is Incorrect:**

**A. Cockayne Syndrome:** This is an autosomal recessive disorder affecting DNA repair mechanisms. Unlike XP, it primarily affects transcription and DNA replication processes, leading to developmental abnormalities and neurological issues, but does not involve skin cancers.

**B. Celiac Disease:** This is an autoimmune disorder affecting the gastrointestinal tract, causing inflammation and damage to the small intestine's mucosal lining. It does not involve skin cancer or nucleotide excision repair mechanisms.

**C. Celiac Disease:** Similar to option B, celiac disease is an autoimmune disorder affecting the gastrointestinal tract, not skin cancers or nucleotide excision repair mechanisms.

**D. Niemann-Pick Disease:** This is a lipid storage disorder affecting lysosomes, leading to intracellular cholesterol accumulation. It does not relate to nucleotide excision repair mechanisms or skin cancers.

**Clinical Pearl:** XP patients should avoid sunlight exposure, wear protective clothing, and use sunscreens with high SPF (Sun Protection Factor) to minimize skin cancer risk. Early diagnosis and treatment of skin cancers are crucial in improving survival rates.