**Core Concept**
Prions are misfolded proteins that cause infectious diseases in animals, including humans. They are unique in that they are composed entirely of protein material, lacking nucleic acids (DNA or RNA). This characteristic makes prions resistant to various methods of inactivation and allows them to be highly infectious.

**Why the Correct Answer is Right**
The correct answer is **D. Protein only**. Prions are composed solely of protein, which allows them to maintain their infectious conformation and transmit their misfolded structure to other proteins. This process of protein misfolding and transmission is the hallmark of prion diseases, such as Creutzfeldt-Jakob disease (CJD) in humans and scrapie in sheep.

**Why Each Wrong Option is Incorrect**
**Option A:** Incorrect because prions do contain some protein material, but they also lack nucleic acids. The statement is incomplete and misleading.
**Option B:** Incorrect because prions are not composed of lipids, which are a major component of cell membranes. Prions are primarily proteinaceous in nature.
**Option C:** Incorrect because prions are not composed of nucleic acids (DNA or RNA). The presence of nucleic acids is a characteristic that distinguishes prions from other types of infectious agents.

**Clinical Pearl / High-Yield Fact**
Prion diseases are transmissible and can cause significant neurological damage, leading to symptoms such as dementia, memory loss, and coordination problems. The unique properties of prions make them difficult to diagnose and treat, and current treatments are largely supportive and aimed at managing symptoms.

**Correct Answer: D. Protein only**