Read Assertion and Reason and select correct option:
Assertion (A): Phenylketonuria is a recessive hereditary disease caused by body's failure to oxidize an amino acid phenylalanine to tyrosine because of defective enzyme.
Reason (R): It results in the presence of phenylalanine in urine.

Question

Read Assertion and Reason and select correct option: 
Assertion (A): Phenylketonuria is a recessive hereditary disease caused by body's failure to oxidize an amino acid phenylalanine to tyrosine because of defective enzyme. 
Reason (R): It results in the presence of phenylalanine in urine.

Accepted Answer

The correct option is 3: (A) is true but (R) is false.

This is an Assertion-Reason question that requires a precise understanding of the biochemical pathology and clinical features of Phenylketonuria (PKU). **Assertion (A)** is a correct and fundamental biological fact. PKU is indeed an **autosomal recessive** hereditary disease, meaning an individual must inherit a defective gene from both parents to express the disorder. The metabolic failure is caused by a defect in the gene for the enzyme Phenylalanine Hydroxylase (PAH), which is necessary to oxidize the amino acid **phenylalanine** into its required product, **tyrosine**. The failure of this enzyme causes phenylalanine to accumulate to toxic levels, leading to the severe symptoms of untreated PKU. However, **Reason (R)** is factually false in its specific terminology. The condition is called Phenyl*keton*uria because the accumulated phenylalanine is shunted to an alternate metabolic pathway, resulting in the production and excretion of various toxic byproducts, which are collectively known as **phenylketones** (such as phenylpyruvic acid, phenylacetic acid, and phenyllactic acid). Therefore, the disease results in the presence of *phenylketones* in the urine, not primarily the amino acid phenylalanine itself, rendering the Reason (R) statement technically incorrect, even though the overall pathology is related.

The incorrect option is 1: Both (A) and (R) are true and (R) is correct explanation of (A).
This option fails because it incorrectly claims the Reason (R) is a true statement. As explained, the definitive chemical signature found in the urine that gives the disease its name is the presence of phenylketones, not phenylalanine. Furthermore, even if both statements were true, the excretion of a substance in the urine (R) is merely an *effect* or *symptom* of the primary metabolic cause (A), and therefore does not serve as the correct biochemical explanation for the enzyme defect described in the assertion. The true explanation for (A) would be the specific gene mutation leading to the non-functional enzyme.

The incorrect option is 2: Both (A) and (R) are true, but (R) is not the correct explanation of (A).
This option must be rejected because its premise—that both the Assertion (A) and the Reason (R) are true—is flawed. The Assertion (A) is scientifically accurate, describing the recessive inheritance and the enzyme deficiency that prevents the conversion of phenylalanine to tyrosine. However, Reason (R) is incorrect because it specifies the presence of 'phenylalanine' in the urine, when the disease is named after, and diagnosed by, the presence of its ketone derivatives, the 'phenylketones'. Therefore, as Reason (R) is false, this option is invalid regardless of the explanatory power.

The incorrect option is 4: Both (A) and (R) are false.
This option is incorrect because the central statement, **Assertion (A)**, is unequivocally true. Phenylketonuria is a classic example of an autosomal recessive single-gene disorder where a defective enzyme causes the failure to metabolize the amino acid phenylalanine, leading to severe pathology if untreated. Since Assertion (A) correctly states the genetic cause and metabolic defect of the disease, any option that claims it is false cannot be the correct answer. The fundamental details of PKU's etiology are well-established in the field of molecular genetics.