Phenylketonuria (PKU) is a genetic disorder caused by a mutation that prevents the breakdown of phenylalanine. This allows phenylalanine levels to accumulate in the blood and brain, which can cause intellectual disabilities if left untreated. The condition is usually diagnosed via newborn screening, which tests for high phenylalanine levels. Treatment requires a lifelong low-phenylalanine diet to prevent cognitive issues and other health problems associated with elevated phenylalanine. Strict dietary control is necessary to manage PKU and avoid complications.