Understanding Phenylketonuria (PKU)

Phenylketonuria (PKU) is a genetic disorder that affects how the body processes the amino acid phenylalanine. Let's analyze the given statements to identify which ones are true regarding Phenylketonuria.

Analyzing Each Statement about Phenylketonuria

We will now look at each statement individually:

• (A) Phenylketonuria is a sex-linked disorder
  This statement is incorrect. Phenylketonuria is an autosomal recessive disorder. This means the gene responsible for PKU is located on an autosome (one of the non-sex chromosomes), and an individual must inherit two copies of the mutated gene (one from each parent) to be affected. Sex-linked disorders are carried on the X or Y chromosome.
• (B) Phenylpyruvic acid is accumulated and converted to phenylalanine
  This statement is incorrect. In PKU, the enzyme needed to break down phenylalanine is deficient. As a result, phenylalanine accumulates in the blood and tissues. Excess phenylalanine is then converted into other substances, including phenylpyruvic acid. So, it is phenylalanine that is converted to phenylpyruvic acid, not the other way around, and phenylalanine is the primary substance that accumulates and causes problems.
• (C) It is caused by a mutation in a pleiotropic gene
  This statement is correct. The gene responsible for PKU is the PAH gene, which codes for the enzyme phenylalanine hydroxylase. A mutation in this single gene can affect multiple, seemingly unrelated traits or systems in the body. For example, PKU can lead to intellectual disability, lighter skin and hair color (due to impaired melanin production, which requires tyrosine, a product of phenylalanine metabolism), and a musty odor in sweat and urine (due to the accumulation of phenylketones like phenylpyruvic acid). This single gene affecting multiple traits is a characteristic of pleiotropy.
• (D) The affected individual lacks enzyme phenylalanine hydroxylase
  This statement is correct. Phenylketonuria is primarily caused by mutations in the PAH gene, leading to a deficiency or complete lack of the enzyme phenylalanine hydroxylase (PAH). This enzyme is crucial for converting phenylalanine into tyrosine. Without functional PAH, phenylalanine builds up to toxic levels in the body.

Identifying the Correct Statements

Based on our analysis:

• Statement (A) is false.
• Statement (B) is false.
• Statement (C) is true.
• Statement (D) is true.

Therefore, the statements that hold true for Phenylketonuria are (C) and (D).

Selecting the Correct Option

Let's check the given options to find the one that includes only statements (C) and (D).

The option that correctly identifies statements (C) and (D) as true is option (c).

Revision Table: Key Facts about Phenylketonuria

Additional Information: Genetic Disorders and Metabolism

Genetic disorders can be classified based on their inheritance pattern, such as autosomal dominant, autosomal recessive, X-linked dominant, X-linked recessive, and mitochondrial inheritance. Phenylketonuria falls under autosomal recessive inheritance.

Metabolic disorders, like PKU, occur when there is a defect in a specific enzyme or metabolic pathway. In the case of PKU, the defect is in the pathway that metabolizes the amino acid phenylalanine. Amino acids are the building blocks of proteins. Phenylalanine is one of the essential amino acids, meaning it must be obtained from the diet.

The conversion of phenylalanine to tyrosine is a critical step. Tyrosine is needed for the synthesis of proteins, thyroid hormones, and melanin (the pigment responsible for skin and hair color). When phenylalanine cannot be converted, tyrosine becomes a conditionally essential amino acid, and its deficiency can contribute to some of the symptoms.

The accumulation of phenylalanine and its breakdown products like phenylpyruvic acid are toxic to the central nervous system, especially during development. This is why early diagnosis through newborn screening and strict dietary management (low phenylalanine diet) are crucial to prevent severe intellectual disability and other complications.