**Core Concept**
Prion diseases, including Creutzfeldt-Jakob disease (CJD), are characterized by the misfolding of prion proteins (PrP), leading to their aggregation and formation of amyloid fibrils. The secondary structure of these misfolded proteins plays a crucial role in the pathogenesis of prion diseases.

**Why the Correct Answer is Right**
The secondary structure of prion proteins in prion diseases like CJD is primarily composed of **beta-sheets**. This is in contrast to the normal, cellular prion protein, which has a predominantly **alpha-helical** secondary structure. The conversion of the normal prion protein to the misfolded, beta-sheet-rich form is thought to be the key event in the pathogenesis of prion diseases. This conversion is mediated by the interactions between the misfolded prion protein and the normal prion protein, leading to the formation of a prion seed that can propagate the misfolding process.

**Why Each Wrong Option is Incorrect**
* **Option A:** The secondary structure of prion proteins in prion diseases is not primarily composed of alpha-helices, which are characteristic of the normal prion protein.
* **Option B:** While the misfolded prion protein does contain beta-sheets, the correct answer is more specific and accurate in describing the secondary structure of prion proteins in prion diseases.
* **Option D:** The secondary structure of prion proteins in prion diseases is not primarily composed of random coils or loops.

**Clinical Pearl / High-Yield Fact**
Prion diseases, including CJD, are a group of fatal neurodegenerative disorders that are characterized by the misfolding of prion proteins. The misfolded prion protein is resistant to degradation by proteases and can propagate its misfolding to other normal prion proteins, leading to the accumulation of amyloid fibrils in the brain and the characteristic symptoms of prion disease.

**Correct Answer:** C. beta-sheets