In PKU, what happens to phenylalanine levels if the condition is not treated?

• A. They decrease
• B. They stay normal
• C. They increase
• D. They fluctuate

Answer: (C)

When PKU is not treated, phenylalanine levels rise because the body cannot efficiently convert phenylalanine to tyrosine due to deficiency of the enzyme phenylalanine hydroxylase. This blocks the metabolic pathway, so phenylalanine from diet accumulates in blood and tissues. With ongoing intake and the blocked conversion, levels increase and can reach toxic concentrations that harm brain development. Early management aims to keep phenylalanine low by restricting intake and providing tyrosine, sometimes with other therapies for certain patients. The other options don’t fit because untreated PKU does not typically keep levels normal or cause a spontaneous decrease; fluctuations may occur with variable intake, but the hallmark is a sustained rise in phenylalanine.