21st aminoacid is –
**Core Concept**
The question refers to the concept of amino acids, which are the building blocks of proteins. The human body uses 20 standard amino acids to synthesize proteins, but there are a few exceptions and additional amino acids that are considered.
**Why the Correct Answer is Right**
The correct answer is not one of the standard 20 amino acids. Instead, it is a modified or non-standard amino acid that plays a crucial role in various biological processes. Selenocysteine is the 21st amino acid, incorporated into proteins in response to specific codons and is a crucial component of enzymes that contain selenium.
**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not refer to a known amino acid. There are 20 standard amino acids, and any additional amino acids would be considered non-standard.
* **Option B:** This option is incorrect because it refers to a different type of molecule, not an amino acid. Glycine is a standard amino acid, but this option is likely referring to a different compound.
* **Option C:** This option is incorrect because it is not a recognized amino acid. This option may be a distractor or a made-up compound.
**Clinical Pearl / High-Yield Fact**
Selenocysteine is an essential amino acid for certain enzymes, including glutathione peroxidase, which protects cells from oxidative damage.
**Correct Answer:** C. Selenocysteine.