The exact cause of many of the psychiatric disorders is unknown, but the phenylketonuria (PKU) is an exception.
PKU is a metabolic genetic disorder that causes elevated levels of phenylalanine in the blood. The PKU is a homozygous recessive condition caused due to the mutation in PAH (phenylalanine hydroxylase gene). In these patients, phenylalanine is not converted into tyrosine, so it accumulates in the body. PKU leads to mental disorders.
Thus, the blank can be filled with Phenylketonuria.