Children with phenylketonuria usually start showing symptoms within 3 to 6 months after birth: Firstly, the nervous system is affected, which can be seen in developmental delays in intelligence, often with IQs lower than normal. Symptoms may include restlessness, depression, hyperactivity, or minor epileptic seizures. A few may exhibit increased muscle tone or heightened tendon reflexes. A few months after birth, due to insufficient melanin synthesis, the skin and hair undergo changes. Hair color may shift from black to yellow, the skin may appear very pale, and eczema is also more common. Additionally, because of the excretion of greater amounts of phenylacetic acids in urine and sweat, there may be a distinct mouse-like urine odor.