Which of the following is an example of pleiotropy?

The underlying mechanism of pleiotropy is the effect of a gene on metabolic pathways which contribute towards different phenotypes.  An example of this is the disease phenylketonuria, which occurs in humans. The disease is caused by mutation in the gene that codes for the enzyme phenyl alanine hydroxylase (single gene mutation). This manifests itself through phenotypic expression characterised by mental retardation and a reduction in hair and skin pigmentation.