An individual develops intellectual disability because of a disorder called phenylketonuria. Which of the following occurrences is a cause of the intellectual disability in the individual?

A) The replacement of myelin by hard, fibrous tissue in the individual's neurons

B) The inhibition of normal myelin development in the individual's brain

C) Insulation of the neurons by myelin

D) Myelination of the prefrontal matter of the individual's brain

B) The inhibition of normal myelin development in the individual's brain

Explanation:

Phenylketonuria is a disorder characterized by abnormal levels of phenylalanine in the body. It is caused by mutation in DNA that leads to the deficiency of an enzyme known as phenylalanine hydroxylase, which converts phenylalanine to tyrosine, a fundamental substance for dopamine production - related to working memory.

Some of the symptoms of this disorder is intellectual disability, which is caused mainly by demyelination or defective myelin biosynthesis, leading to the inhibition of myelin, a fatty insulating sheath that protects nerves.

Therefore, the cause of the intellectual disability in the individual is the inhibition of normal myelin development in the individual's brain.