Correct Answer: (c) Inherited lack of an enzyme
Solution:Phenylketonuria (PKU) is a disease caused by a metabolic disorder inherited as a recessive trait. The dominant gene 'P', in this case, codes for the enzyme, phenylalanine hydroxylase, formed in the liver cells. This enzyme catalyzes conversion of phenylalanine to tyrosine. In homozygous recessive genotypes, the absence of this enzyme causes a high level of phenylalanine in blood and tissues fluids. The phenotypic effects include a progressive mental retardation starting a few month after birth, seizures, and anomalies of teeth enamel and bones.