Phenylketonuria severity is generally determined by clinical manifestations and blood phenylalanine concentration. Mild phenylketonuria is characterized by mild or asymptomatic intelligence, motor, and developmental delays, light hair and skin color, and a musty odor in urine and sweat. It includes clinical features such as seizures accompanied by abnormal mental and behavioral symptoms, with blood phenylalanine levels less than 120 to 360 micromoles per liter. This is primarily seen in a very small number of newborns or premature infants, or in cases where there is higher residual activity of phenylalanine hydroxylase enzyme.