**Question:** Keratin contains -

**Core Concept:** Keratin is a structural protein that forms the hard, protective outer layer of the skin called the stratum corneum and hair shafts. It plays a crucial role in maintaining skin integrity and resisting mechanical stress.

**Why the Correct Answer is Right:** Keratin is primarily composed of proteins, specifically alpha-keratin, which is a type of intermediate filament protein. Alpha-keratin is composed of a repeating sequence of amino acids, including cysteine, which forms disulfide bridges. These bridges, along with the protein's hierarchical structure, give keratin its characteristic strength and rigidity.

**Why Each Wrong Option is Incorrect:**

A. This option is incorrect because keratin is not composed of glycoproteins. Glycoproteins contain both proteins and carbohydrates, while keratin is primarily composed of proteins.

B. This option is incorrect because lipids are not a primary component of keratin. Keratin primarily consists of proteins, specifically alpha-keratin, which forms the structural backbone of the protein.

C. This option is incorrect because nucleic acids are not a component of keratin. Keratin is composed of proteins, and nucleic acids are involved in genetic information storage and replication.

D. This option is incorrect because carbohydrates are not the primary component of keratin. While keratinocytes do produce glycoproteins, keratin itself is primarily composed of proteins.

**Clinical Pearl:** In clinical practice, understanding the composition of keratin helps differentiate between different types of skin disorders. For example, in epidermolysis bullosa, a group of genetic skin disorders, there is a deficiency or mutation in keratin proteins, leading to blistering and fragility of the skin.

**Correct Answer:** D. keratin is composed of proteins