**Question:** Ultraviolet light can damage a DNA strand causing:

**Core Concept:** Ultraviolet (UV) radiation is a type of electromagnetic radiation that falls within the ultraviolet range of the light spectrum. When UV light interacts with the DNA molecule, it can cause damage to the genetic material, leading to various biological effects.

**Why the Correct Answer is Right:** UV radiation can cause damage to DNA strands through a process called photodamage. The correct answer, **C. Formation of pyrimidine dimers**, refers to a specific type of photodamage that occurs when UV light reacts with pyrimidine bases (cytosine, thymine, and uracil) in DNA. This reaction forms a covalent bond between adjacent pyrimidine bases, resulting in the formation of pyrimidine dimers.

**Why Each Wrong Option is Incorrect:**

A. **Formation of purine bases dimers (guanine and adenine)** is an incorrect answer because purine bases (guanine and adenine) do not react with UV light to form dimers.

B. **DNA excision repair** is a cellular response to DNA damage, not a direct result of UV light exposure.

D. **Increased mutation rate and cancer risk** are indirect consequences of DNA damage due to UV light, not a direct result of the damage itself.

**Clinical Pearl / High-Yield Fact:** DNA damage caused by UV radiation is a significant concern in the field of dermatology, particularly in photodermatology. Photodamage contributes to skin aging, skin cancer, and other skin disorders. Maintaining adequate sun protection, wearing sunscreen and protective clothing, and limiting sun exposure are essential for skin health and prevention of these complications.