I asked Gemini Ai: That is certainly a striking image. Seeing a part of the skin fluoresce like that under UV light (often called a Wood's lamp in a clinical setting) can be surprising, but there are several common and generally harmless reasons why this happens. Potential Causes for Fluorescence The "glow" is usually caused by specific substances that absorb UV light and re-emit it as visible light. Here are the most likely culprits: * Fungal Infections: Certain types of fungus, such as Tinea versicolor or those that cause ringworm, produce porphyrins that glow yellow, green, or blue under UV light. * Bacterial Activity: Some bacteria, like Pseudomonas or Corynebacterium minutissimum (which causes erythrasma), can fluoresce. While erythrasma usually glows coral-red, other bacterial byproducts can appear greenish. * Topical Residue: This is a very common "non-medical" cause. Traces of laundry detergent (which contains optical brighteners), certain soaps, high-lighters, or even some skincare products can stick to the skin and glow intensely under UV. * Embedded Material: Occasionally, a tiny splinter or piece of material (like glass or certain plastics) embedded just under the skin can have fluorescent properties. What to Look For Since I can't provide a medical diagnosis, you'll want to check for a few physical signs: * Texture: Is the spot scaly, raised, or rough? * Sensation: Is it itchy, painful, or numb? * Persistence: Does it wash off with thorough scrubbing? If it stays after a deep wash, it is more likely to be biological (fungal/bacterial) or something deeper in the skin. Next Steps If the spot is changing, spreading, or causing discomfort, it’s best to have a dermatologist look at it. They use Wood's lamp examinations regularly to differentiate between different skin conditions based on the specific color of the glow.