**Core Concept**
Tuberous sclerosis is a genetic disorder characterized by the growth of non-cancerous tumors in various parts of the body, including the skin. The condition leads to the formation of hypomelanotic macules, ash-leaf spots, shagreen patches, and other skin lesions. The wood lamp examination is a diagnostic tool that helps identify these lesions by highlighting their color under ultraviolet light.

**Why the Correct Answer is Right**
The correct answer is related to the characteristic color of ash-leaf spots under a wood lamp examination. Ash-leaf spots are a type of hypomelanotic macule that appears as a light brown or pale yellowish spot on the skin. When exposed to ultraviolet light, these spots typically fluoresce with a bright yellowish or greenish color due to the reduction of melanin in the affected skin. This phenomenon is a result of the reduced melanin content in the skin, which allows the ultraviolet light to penetrate deeper and cause the characteristic fluorescence.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it does not accurately describe the color of ash-leaf spots under a wood lamp examination. Ash-leaf spots do not typically appear as a dark brown or black color under ultraviolet light.
**Option B:** This option is incorrect because it does not accurately describe the color of ash-leaf spots under a wood lamp examination. Ash-leaf spots do not typically appear as a reddish or purplish color under ultraviolet light.
**Option C:** This option is incorrect because it does not accurately describe the color of ash-leaf spots under a wood lamp examination. Ash-leaf spots do not typically appear as a blue or violet color under ultraviolet light.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the wood lamp examination can help identify various skin lesions, including those associated with tuberous sclerosis. This diagnostic tool can aid in the early detection of the condition, allowing for timely intervention and management.

**Correct Answer: C. Yellowish or greenish.**