mineli lu at children's hospital, los angeles. >> in savannah's case we can see that she has developed red welts on her hand as well as her fingers. >> reporter: and this along with her young age, made dr. lu suspect savannah had arythropoietic porphyria. it was featured on "house." >> look at her arm. >> reporter: but could this be what was affecting savannah? >> the challenge is body how rare the condition is as well as the subtlety sometimes. there may be some red patches, maybe areas that look sunburned, that can lead to a long lag in diagnosis. >> reporter: after years of no answers, a simple blood test finally gave them one. >> what did the doctors finally tell you? >> that she had -- say it. >> erythropoirtic porypheria. >> reporter: it would mean a life sentence confined to a world of shadows. >> how do you explain this to your friends? >> that i'm allergic to the sun, i can't be out, i can never be at the beach, can never go to a sunlit pool. literally i have to be in the dark. dark's my home. >> reporter: savannah now 11 years old, has to wait until evening to swim in her poo