Phenylketonuria is best described as which of the following?

• A. A genetic error of metabolism that disrupts myelination
• B. An autoimmune disease causing demyelination
• C. A chromosomal deletion resulting in cognitive impairment
• D. A progressive neurodegenerative disorder

Answer: (A)

Phenylketonuria is a genetic metabolic disorder caused by mutations in the gene for phenylalanine hydroxylase, leading to an inability to convert phenylalanine to tyrosine. This causes elevated phenylalanine levels that, if not treated in infancy, interfere with brain development and can disrupt myelin formation in the central nervous system. It is inherited in an autosomal recessive pattern, making it a genetic error of metabolism rather than an autoimmune process or a chromosomal deletion. It is not a progressive neurodegenerative disorder; rather, the major issues arise from metabolic toxicity during development, which can be prevented or minimized with early detection and dietary management that limits phenylalanine intake. The other descriptions don’t fit PKU because they describe autoimmune demyelination, a chromosomal deletion, or a progressive degenerative course, none of which capture the genetic enzymatic defect and its treatable metabolic nature.