What characterizes the condition known as PKU?

Phenylketonuria (PKU) is characterized by an accumulation of phenylalanine in the body due to a deficiency in the enzyme phenylalanine hydroxylase. This enzyme is responsible for converting phenylalanine to tyrosine. When phenylalanine accumulates, it can lead to an overflow of this amino acid in the urine, a condition termed overflow aminoaciduria.

This accumulation occurs because, in PKU, the body cannot effectively metabolize phenylalanine, leading to increased levels in both blood and urine. If left untreated, elevated phenylalanine can cause serious neurological damage and developmental delays.

The other options do not correctly reflect the nature of PKU. For instance, PKU does not involve an excess production of tyrosine, nor is it primarily related to a deficiency of protein intake or blood urea nitrogen overload. Instead, the hallmark of the condition is the inability to process phenylalanine, resulting in its buildup and subsequent excretion in the urine.