A 14-year-old boy goes to the doctor with complaints of tiredness. He's an extremely picky eater. (Think a daily diet of French fries, plus snacking on Pringles potato chips, white bread and some processed pork.) But overall, he appears OK. He's not overweight and takes no medications.
Tests show he has anemia and low levels of vitamin B12, so he's given B12 injections and diet advice. But a year later, he has begun to lose his vision. Then, by age 17, he's legally blind.