**Core Concept**
Keratin is a type of structural protein found in skin, hair, and nails. The unique properties of keratin in skin and nails are attributed to the presence of specific chemical bonds. Understanding the types of chemical bonds present in keratin is essential for appreciating its distinct characteristics.

**Why the Correct Answer is Right**
The primary difference in consistency between skin and nail keratin is due to the presence of disulfide bonds in nail keratin. Disulfide bonds are covalent bonds formed between sulfur atoms of cysteine residues in the keratin protein. These bonds are responsible for the rigidity and strength of nail keratin, making it more resistant to wear and tear compared to skin keratin. In contrast, skin keratin contains more disulfide bridges, but also has a higher proportion of other amino acids such as serine and glycine, which contribute to its flexibility and elasticity.

**Why Each Wrong Option is Incorrect**
**Option A:** Hydrogen bonds are important for the stability of proteins, but they do not account for the difference in consistency between skin and nail keratin.

**Option B:** Ionic bonds are not the primary type of bond responsible for the distinct properties of keratin in skin and nails.

**Option C:** Van der Waals forces are weak intermolecular forces that contribute to the overall structure of proteins, but they are not the key factor in determining the consistency of keratin in skin and nails.

**Clinical Pearl / High-Yield Fact**
To remember the key difference between skin and nail keratin, recall that disulfide bonds are like strong "glue" that holds nail keratin together, making it more rigid and resistant to wear.

**Correct Answer: C. Disulfide bonds.**