Which of the following statements concerning phenylketonuria (PKU) is true? A. For those affected, dietary restrictions are critical later in life. B. It is interesting to researchers because it does not require an interaction of genes and environment to be expressed. C. Left untreated, it can cause intellectual disability. D. It manifests due to excessive intake of fatty foods.

The correct answer would be C. Left untreated, it can cause intellectual disability.

Phenylketonuria (PKU) is a rare autosomal recessive disorder caused by a defect in a gene responsible for making phenylalanine digesting enzyme.

This defect leads to the lack or deficiency of this enzyme in the body. Consequently, phenylalanine builds up in the body.

People with this disorder are usually put on a restricted diet which restricts protein-rich food (such as milk, meat, cheese etc as they contain amino acid phenylalanine) from their meals.

Untreated PKU can cause various complications such as brain damage (intellectual disability), neurological problems, major health and development problems etc.