Subsequently, high levels of phenylalanine and two closely related phenylalanine derivatives develop. These compounds are toxic to the central nervous system and cause brain damage. Damage to the brain causes marked mental retardation by the end of the first year of life. These children may have an odor described as mouse-like that results from the accumulation of phenylacetic acid. This odor may be detected on their breath, skin, and urine.
Genetic counseling is recommended for prospective parents with a family history of phenylketonuria.
The carrier state for PKU can be detected by enzyme assays and PKU can be diagnosed prenatally. Screening immediately after birth is done routinely in all U.S. hospitals. Strict compliance to the diet is necessary to reduce or prevent mental retardation . Phenylalanine occurs in significant amounts in milk, eggs, and other common foods. It can be used throughout life as a protein source that is extremely low in phenylalanine and balanced for the remaining essential amino acids .