**Question:** Amino acid produced by adding hydroxyl group to phenyl side chain of phenylalanine

**Core Concept:** Amino acids are the building blocks of proteins, and some of them undergo modifications, such as hydroxylation, to form new molecules with specific functions. Phenylalanine is an essential amino acid that can be hydroxylated to form tyrosine.

**Why the Correct Answer is Right:** Tyrosine is the amino acid produced when the hydroxyl group (OH) is added to the phenyl group of phenylalanine. This process is known as hydroxylation and is catalyzed by enzymes called hydroxylases. In this case, the relevant enzyme is the phenylalanine hydroxylase, which adds the hydroxyl group to the phenylalanine molecule, resulting in tyrosine.

**Why Each Wrong Option is Incorrect:**

A. Tyrosine is not produced from the addition of hydroxyl group to the side chain of valine. Valine is another essential amino acid, and its hydroxylation would result in an amino acid different from tyrosine.
B. Tyrosine is not produced from the addition of hydroxyl group to the side chain of leucine. Leucine is another essential amino acid, and its hydroxylation would result in an amino acid different from tyrosine.
C. Tyrosine is not produced from the addition of hydroxyl group to the side chain of isoleucine. Isoleucine is another essential amino acid, and its hydroxylation would result in an amino acid different from tyrosine.
D. Tyrosine is not produced from the addition of hydroxyl group to the side chain of methionine. Methionine is an essential amino acid, but its hydroxylation would result in an amino acid different from tyrosine.

**Correct Answer:** D. Tyrosine is the amino acid produced when the hydroxyl group (OH) is added to the phenyl group of phenylalanine. This process occurs in the liver and is essential for the synthesis of several important molecules, including dopamine, norepinephrine, and epinephrine.

**Clinical Significance:** The hydroxylation of phenylalanine to tyrosine is a critical step in the synthesis of catecholamines, which are neurotransmitters involved in regulating autonomic functions, mood, and cognitive processes. Dysfunction in catecholamine synthesis can lead to various medical conditions, such as Phenylketonuria (PKU), a genetic disorder caused by a deficiency in phenylalanine hydroxylase enzyme. In PKU, phenylalanine accumulates in the blood, leading to neurodevelopmental issues if left untreated.

**Why This is a Core Concept in Medicine:** Understanding the hydroxylation of phenylalanine to tyrosine is crucial in understanding the synthesis of catecholamines and their roles in the body. This knowledge is essential for diagnosing and treating conditions like PKU and other disorders affecting catecholamine synthesis.