A rare genetic disorder that makes sunlight excruciatingly painful — caused by a deficiency in a blood enzyme that leaves a toxic compound accumulating in skin.Deficiency of ferrochelatase leads to accumulation of protoporphyrin IX in red blood cells, plasma, and skin. Protoporphyrin IX absorbs visible light (not just UV) and releases the energy as reactive oxygen species, causing severe burning pain within minutes of sun exposure. Afamelanotide (Scenesse) is FDA-approved specifically for EPP — by stimulating melanogenesis, the induced pigmentation absorbs light before it reaches protoporphyrin IX. This is a rare disease approval, not a cosmetic tanning approval.