In which of the following diseases does the affected individual lack…

The correct answer is Option (3) → Phenylketonuria

Phenylketonuria (PKU) is an inborn error of metabolism inherited as an autosomal recessive trait. The affected person lacks the enzyme that converts phenylalanine into tyrosine, so phenylalanine collects and is changed into phenylpyruvic acid and other derivatives, which harm the brain.

Option (1) Thalassemia — Incorrect. Thalassemia is an autosome-linked recessive blood disease in which too few globin molecules are made. It is not caused by a missing enzyme of amino acid metabolism.

Option (2) Sickle-cell anaemia — Incorrect. Sickle-cell anaemia is caused by the substitution of glutamic acid by valine in the β globin chain, which makes a faulty haemoglobin.

Option (3) Phenylketonuria — Correct. In PKU the enzyme that converts phenylalanine into tyrosine is missing, so phenylalanine and its derivatives accumulate.

Option (4) Haemophilia — Incorrect. Haemophilia is a sex-linked recessive disease in which a protein of the blood-clotting cascade is affected, so a simple cut causes non-stop bleeding.

NCERT Extract:
Mendelian Disorders – “… Phenylketonuria : This inborn error of metabolism is also inherited as the autosomal recessive trait. The affected individual lacks an enzyme that converts the amino acid phenylalanine into tyrosine. As a result of this phenylalanine is accumulated and converted into phenylpyruvic acid and other derivatives. Accumulation of these in brain results in mental retardation.”