Phenylalanemia I is due to deficiency of ?
**Core Concept**
Phenylalanemia I, also known as phenylketonuria (PKU), is a genetic disorder caused by a deficiency of the enzyme phenylalanine hydroxylase (PAH). This enzyme is crucial for the conversion of the amino acid phenylalanine (Phe) to tyrosine (Tyr) in the liver.
**Why the Correct Answer is Right**
The deficiency of PAH leads to the accumulation of phenylalanine in the body, causing a range of clinical symptoms, including intellectual disability, seizures, and behavioral problems. PAH is a mitochondrial enzyme that catalyzes the hydroxylation of phenylalanine to tyrosine, a reaction that is essential for the proper functioning of the brain and other organs. The absence of this enzyme results in the toxic buildup of phenylalanine, leading to the development of phenylalanemia I.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the deficiency of dihydropteridine reductase (DHPR) leads to a separate disorder known as dihydropteridine reductase deficiency, which is different from phenylalanemia I.
* **Option B:** This option is incorrect because the deficiency of tyrosine aminotransferase (TAT) leads to a different disorder known as tyrosinemia type II, which is characterized by the accumulation of tyrosine and its metabolites.
* **Option D:** This option is incorrect because the deficiency of homogentisate 1,2-dioxygenase (HGD) leads to a disorder known as alkaptonuria, which is characterized by the accumulation of homogentisic acid and its metabolites.
**Clinical Pearl / High-Yield Fact**
Phenylalanemia I is an autosomal recessive disorder, meaning that a person must inherit a defective gene from both parents to develop the condition. Early diagnosis and treatment with a phenylalanine-restricted diet can significantly improve outcomes and prevent intellectual disability.
**Correct Answer: C. Phenylalanine Hydroxylase (PAH)**