Phenylketonuria (PKU) is characterized by which biochemical problem?

• A. Autoimmune reaction
• B. Defect in glucose metabolism
• C. Inability to convert phenylalanine to tyrosine
• D. Excess tyrosine causing brain damage

Answer: (C)

Phenylketonuria is a metabolic condition caused by deficiency of the enzyme phenylalanine hydroxylase (or its cofactor), so phenylalanine cannot be converted into tyrosine. Because this conversion is blocked, phenylalanine builds up in the blood and brain, while tyrosine becomes deficient, leading to neurodevelopmental problems if not treated. This is why the correct idea is an inability to convert phenylalanine to tyrosine. The other options describe processes that aren’t involved in PKU—an autoimmune reaction, a defect in glucose metabolism, or excess tyrosine causing damage—so they don’t fit the underlying mechanism. Management focuses on restricting phenylalanine intake and providing tyrosine, with newborn screening helping to prevent intellectual disability by addressing the issue early.