Phenylketonuria, or PKU as it is more familiarly known, is a rare disorder, affecting only ∼1 in 15 000 people. In the United States, for example, ∼275 infants will be born with the disease each year. Thus in a lifetime of practice most pediatricians will not encounter a single case. Yet probably every pediatrician in the industrialized world has learned about PKU during medical school, many parents vividly remember the heel-stick test for their newborn, and scientists interested in genetics and metabolism say that they hope to “find another PKU.” Why has such a rare condition garnered so much attention?

PKU is famous in part because it is widely seen as a victory for scientific medicine. If the condition is detected in the newborn period and a specialized diet is instituted, the profound cognitive impairment usually caused by PKU is averted. For the diet to be effective, however, the...

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