**Core Concept**
Prions are infectious agents composed of a misfolded protein that can induce normal proteins in the brain to also misfold, leading to neurodegenerative diseases such as Creutzfeldt-Jakob disease in humans, Bovine Spongiform Encephalopathy (BSE) in cattle, and scrapie in sheep. Prions do not contain nucleic acids (DNA or RNA), which makes them unique among infectious agents.

**Why the Correct Answer is Right**
The correct answer consists of **protein**. Prions are composed of a specific type of protein called a prion protein, which is encoded by a normal gene but can misfold into an abnormal structure. This misfolded protein is highly resistant to degradation and can induce other normal prion proteins to also misfold, leading to the characteristic neurodegenerative symptoms of prion diseases.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because prions do not contain nucleic acids, which are characteristic of viruses, bacteria, and other types of infectious agents.
**Option B:** This option is incorrect because prions are not composed of lipids, which are characteristic of viruses and other types of infectious agents.
**Option C:** This option is incorrect because prions are not composed of carbohydrates, which are characteristic of some viruses and other types of infectious agents.

**Clinical Pearl / High-Yield Fact**
Prion diseases are unique in that they can be transmitted through the consumption of contaminated food products, such as beef from cattle infected with BSE, which has led to outbreaks of variant Creutzfeldt-Jakob disease in humans.

**Correct Answer: D. Protein**