The clinical symptoms of phenylketonuria generally include normal appearance at birth, with symptoms usually appearing between three to six months, and becoming more pronounced by the age of one. The most noticeable initial symptoms involve the nervous system; delayed intellectual development is prominent, with intelligence often below normal, along with behavioral abnormalities such as hyperactivity, depression, restlessness, and withdrawal. There could be minor epileptic seizures, and in some cases, increased muscle tone or exaggerated tendon reflexes. Regarding the skin, several months after birth, due to insufficient melanin synthesis, the child’s hair changes from black to yellow, the skin appears pale, and eczema is also relatively common. Additionally, the urine and sweat contain higher levels of phenylacetic acid, which can result in a distinctive mouse-like urine odor.