An essential amino acid is

Options

A. Tryptophan
B. Glycine
C. Glutamine
D. Tyrosine.

Answer

To determine which of the given options is an essential amino acid, we can follow these steps:

### Step 1: Understand Essential Amino Acids
Essential amino acids are those that cannot be synthesized by the body and must be obtained through the diet. There are a total of 20 amino acids, out of which 9 are classified as essential.

### Step 2: Identify the Essential Amino Acids
The essential amino acids include:
1. Valine
2. Leucine
3. Isoleucine
4. Lysine
5. Methionine
6. Histidine
7. Phenylalanine
8. Threonine
9. Tryptophan

### Step 3: Analyze the Options
Now, let's analyze the options given in the question:
1. Tryptophan
2. Glycine
3. Glutamine
4. Tyrosine

### Step 4: Classify Each Option
- **Tryptophan**: This is an essential amino acid.
- **Glycine**: This is a non-essential amino acid.
- **Glutamine**: This is also a non-essential amino acid.
- **Tyrosine**: This is a non-essential amino acid.

### Step 5: Conclusion
Based on the classification, the only essential amino acid from the options provided is **Tryptophan**. Therefore, the correct answer is option 1.

### Final Answer
**Tryptophan** is an essential amino acid.

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