Guthrie card for diagnosing PKU, 2000.

UNITED KINGDOM - JUNE 01: About one in 10,000 children are born with phenylketonuria (PKU). People with PKU are missing an enzyme that breaks down phenylalanine, one of the building blocks of the protein found in a normal diet. It can be serious if left untreated, causing mental impairment. However, PKU can be treated successfully by controlling levels of phenylalanine in the diet. All babies in the UK are now tested for PKU by a heel-prick test shortly after birth, and making use of a Guthrie card. If PKU is diagnosed, babies are immediately given a special diet. (Photo by SSPL/Getty Images)