Introduction Phenylketonuria (usually known as PKU) is a genetic disorder illustrated by the body’s failure to make use of phenylalanine an essential amino acid which is obtained from the diet. Phenylalanine is found in all natural proteins and some artificial sweeteners. Phenylketonuria is normally a Mendelian recessive error of metabolism. The crucial problem in this situation is abnormality of phenylalanine hydroxylase enzyme that leads to an increase in amino acid phenylalanine in the blood serum thus increasing phenylalanine level in the blood up to harmful levels which can cause severe health problems (Howell, 2000). In a classic PKU, phenylalanine levels range between 6-80mg/dl whereas a normal phenylalanine level in blood is 1 mg/dl. If this disease is not identified and treated by consumption of low protein diets, then the amino acid phenylalanine accumulates in the blood leading to mental retardation and a serious brain damage. Causes Phenylketonuria is inherited in a recessive manner whereby the affected person inherited a trait from each parent for the disorder. An individual with one trait does not show phenylketonuria symptoms and is termed as a carrier. Phenylketonuria results from phenylalanine hydroxylase enzyme deficiency, its clinical phenotypes vary and more than 10 mutations in the enzyme gene have been seen in patients with phenylketonuria disease (Henriksen, 1991). The phenylalanine hydroxylase gene (PAH) have the conveyed nucleotide sequence, it encodes the hepatic enzyme phenylalanine hydroxylase). This enzyme hydroxylates phenylalanine an essential amino acid leading to production of another amino acid, tyrosine and the reaction account for elimination of about 75% of phenylalanine. The autosomal disease phenylketonuria (PKU) results from absence of PheOH enzymatic activity because of PAH gene mutations. 99% of the mutant alleles that cause this disease map to the PAH gene and the other 1% maps to several genes that encode enzymes involved in the biosynthesis and regeneration of the cofactors that are involved in hydroxylation. The conversion of phenylalanine to tyrosine does not occur thus permitting the accumulation of phenylalanine in the body resulting to irreversible mental retardation (Erlandsen, 2002). Symptoms Phenylketonuria is characterized by mental retardation, seizures, development of supranuclear motor disturbances, epilepsy, retardation, poor development of tooth and microcephaly. These symptoms can be avoided by phenylalanine‐restricted diet consumption in advance. Neurological analysis of treated patients shows that only minor neurological symptoms like tremor or vigorous deep tendon reflexes show white matter abnormalities. However, patients show symptoms such as vomiting, irritability and a rash like eczema early in life as a result of toxic effect of phenylalanine in the brain and development of neurological symptoms (Lippincott, 1998). The symptoms for phenylketonuria range from mild to severe and the most harsh is the classic PKU and children with classic PKU appear normal when born and these signs develop after a few months. When not treated early, these infants develop permanent brain damage affecting the child’s IQ. Many of these infants have fairer h