**Core Concept**
Tetracycline stains are a well-known side effect of antibiotic therapy, particularly in children whose teeth are still developing. These stains can affect the color and appearance of the teeth, and their visibility can be enhanced under certain conditions.

**Why the Correct Answer is Right**
Tetracycline stains appear fluorescent green under ultraviolet (UV) light due to the binding of tetracycline to hydroxyapatite in tooth enamel. This binding causes an energy transfer that results in the characteristic green fluorescence. The fluorescence is a result of the tetracycline molecule's ability to absorb light in the UV range and emit light at a longer wavelength, which is visible as green fluorescence.

**Why Each Wrong Option is Incorrect**
**Option A:** is incorrect because tetracycline stains do not typically appear yellow under UV light.
**Option B:** is incorrect because tetracycline stains do not typically appear blue under UV light.
**Option C:** is incorrect because tetracycline stains do not typically appear red under UV light.

**Clinical Pearl / High-Yield Fact**
The fluorescence of tetracycline stains under UV light is a useful diagnostic tool for identifying patients who have been exposed to tetracycline during tooth development. This can help clinicians to identify patients who may be at risk for tooth discoloration or other dental abnormalities.

**Correct Answer: D. Green**