Which stain will glow blue-white under ultraviolet light?

• A. Urine
• B. Blood
• C. Oil
• D. Ink

Answer: (A)

Under ultraviolet light, certain substances absorb UV energy and re-emit it as visible light, a property called fluorescence. The color of that glow depends on the chemical makeup of the substance. Urine contains fluorescent compounds like uric acid and related metabolites that produce a bright blue-white glow when excited by UV light. This makes urine stains stand out with a characteristic blue-white fluorescence, which is why it’s the best match for this question.

Blood, oil, and ink can fluoresce as well, but their typical glow colors differ (often orange/red for some blood remnants or yellow-green for many oils, with inks varying by dye). They don’t reliably produce the blue-white glow described here, so urine is the most fitting choice.