**Core Concept**
The child's symptoms suggest a condition known as Xeroderma Pigmentosum (XP), a rare genetic disorder characterized by an extreme sensitivity to ultraviolet (UV) light from the sun or artificial sources. This condition is caused by a deficiency in the DNA repair mechanisms, specifically nucleotide excision repair (NER), which is responsible for repairing DNA damage caused by UV radiation.

**Why the Correct Answer is Right**
In XP, the NER pathway is defective, leading to an accumulation of UV-induced DNA damage, resulting in mutations and genetic instability. This, in turn, causes the development of skin tumors and blisters upon exposure to sunlight. The irregular dark spots on the skin are a result of the abnormal pigmentation due to the accumulation of DNA damage and subsequent mutations in melanocytes. The defective NER pathway is responsible for the inability to repair UV-induced DNA damage, leading to the characteristic symptoms of XP.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not relate to the underlying mechanism of Xeroderma Pigmentosum. While defects in other DNA repair pathways can cause genetic disorders, they do not result in the characteristic symptoms of XP.
* **Option B:** This option is incorrect because it refers to a different genetic disorder, Bloom syndrome, which is characterized by a defect in the BLM gene and an increased sensitivity to UV light, but not the same as XP.
* **Option C:** This option is incorrect because it does not specify a defect in the nucleotide excision repair pathway, which is the underlying cause of XP.
* **Option D:** This option is incorrect because it refers to a different mechanism, not related to the DNA repair pathway.

**Clinical Pearl / High-Yield Fact**
XP is a rare genetic disorder, but it is a classic example of how defects in DNA repair mechanisms can lead to severe consequences, including skin tumors and cancer. It is essential to recognize the characteristic symptoms of XP and to consider this condition in patients with a history of skin sensitivity to sunlight.

**Correct Answer:** C.