Phenylketonuria (PKU) is an inherited disorder, which implies it`s passed down through generations genetically. In this disease, the body lacks the necessary enzyme, known as phenylalanine hydroxylase, to break down a specific amino acid, phenylalanine, leading to its accumulation and potential damage to the brain. Hence, option 3 "inherited lack of an enzyme" rightly defines the cause of PKU.

Option 1, hormonal overproduction, is incorrect as PKU isn`t caused by an excess of any hormone. Similarly, option 2, atrophy of endocrine glands, is unrelated because PKU does not involve the degeneration of any glands. Option 4, non-disjunction, describes a process in which chromosomes fail to separate properly during cell division which is also not the case in PKU. Thus, options 1, 2, and 4 are incorrect.