## Core Concept
Keratin is a type of structural protein that is a key component of the skin, hair, and nails in humans. It is a fibrous protein that provides strength and rigidity to these tissues. The structure and function of keratin are closely related to its amino acid composition.

## Why the Correct Answer is Right
The correct answer, **Cysteine**, is a crucial amino acid in keratin due to its high content of disulfide bonds. These bonds are formed between the thiol groups of cysteine residues, contributing to the stability and rigidity of keratin. This unique property allows keratin to provide mechanical strength to hair, nails, and the outer layer of skin. The disulfide bonds are essential for the alpha-helical and beta-sheet structures that keratin assumes, making it insoluble and tough.

## Why Each Wrong Option is Incorrect
- **Option A:** While various amino acids are present in keratin, **Glycine** is not specifically known for contributing to its unique structural properties like cysteine does. Glycine is the simplest amino acid and is more related to collagen structure.
- **Option B:** **Tryptophan** is an essential amino acid but is not particularly noted for its role in keratin's structure. It is more associated with being a precursor to serotonin and melatonin.
- **Option D:** **Tyrosine** is another amino acid that, while present in many proteins, does not play a specific critical role in the structural integrity of keratin like cysteine.

## Clinical Pearl / High-Yield Fact
A key clinical correlation is that the strength of hair and nails can be affected by the integrity of keratin and, by extension, the availability of cysteine. This is why conditions affecting keratin, such as epidermolysis bullosa simplex, can lead to skin fragility.

## Correct Answer: C. Cysteine