C)Phenyl pyurvic acid

Phenylketonuria is an inborn metabolic disorder in which the individuals lack the hepatic enzyme Phenylalanine hydroxylase located on chromosome 12, which converts phenylalanine into tyrosine.As a result phenylalanine accumulates and gets converted into phenylpyruvic acid,Phenylacetic acid and other derivates which causes severe mental retardation,light pigmentation of skin and hair. And further the phenylpyruvic acid is excreted out in the urine.

**In Phenylketonuria (PKU), the accumulation of which compound in the brain results in mental retardation:**

The compound that accumulates in the brain and causes mental retardation in Phenylketonuria (PKU) is **Phenylalanine**.

**Explanation:**

Phenylketonuria (PKU) is an inherited metabolic disorder characterized by the deficiency or absence of the enzyme phenylalanine hydroxylase (PAH). This enzyme is responsible for converting phenylalanine, an essential amino acid found in dietary proteins, into another amino acid called tyrosine. Without the activity of PAH, phenylalanine cannot be properly metabolized and begins to accumulate in the body, including the brain.

**Effects of Phenylalanine accumulation:**

When phenylalanine accumulates in the brain, it can lead to several detrimental effects, including mental retardation. The excess phenylalanine interferes with normal brain development and function, particularly during the critical period of brain growth in early infancy and childhood.

**Toxic effects of Phenylalanine:**

1. **Disruption of neurotransmitter balance:** Phenylalanine competes with other amino acids, such as tyrosine and tryptophan, for transport across the blood-brain barrier. This competition can disrupt the balance of neurotransmitters in the brain, affecting important processes such as learning, memory, and mood regulation.

2. **Increased levels of phenylalanine metabolites:** Phenylalanine can be metabolized into various byproducts, including phenylpyruvic acid and other toxic compounds. These metabolites are thought to contribute to the neurological damage seen in PKU, leading to mental retardation.

3. **Impaired myelination:** Myelination is the process by which nerve fibers are covered with a protective sheath called myelin. High levels of phenylalanine can interfere with myelin formation and maintenance, leading to disrupted communication between nerve cells and impaired cognitive function.

**Conclusion:**

In Phenylketonuria (PKU), the accumulation of phenylalanine in the brain leads to mental retardation. The toxic effects of phenylalanine, such as disruption of neurotransmitter balance, increased levels of phenylalanine metabolites, and impaired myelination, contribute to the neurological damage observed in PKU. Early diagnosis and dietary management, including a strict phenylalanine-restricted diet, are essential to prevent or minimize the cognitive impairment associated with this condition.