**Core Concept**
The "mad cow" disease, also known as Bovine Spongiform Encephalopathy (BSE), is a neurodegenerative disorder caused by a misfolded protein, specifically a prion. Prions are infectious proteins that can induce normal proteins in the brain to also misfold, leading to brain damage and the characteristic symptoms of BSE.

**Why the Correct Answer is Right**
BSE is caused by the prion protein (PrP) in cattle, which is transmitted through the consumption of contaminated animal feed. The prion protein is highly resistant to heat, radiation, and chemicals, making it difficult to inactivate. When a healthy cattle ingests feed contaminated with the misfolded prion protein, it can cause the normal prion protein in the animal's brain to misfold, leading to the development of BSE.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because BSE is not caused by a virus, such as rabies or bovine leukemia virus.
* **Option B:** This option is incorrect because BSE is not caused by a bacteria, such as E. coli or Salmonella.
* **Option C:** This option is incorrect because BSE is not caused by a fungus, such as Aspergillus or Candida.

**Clinical Pearl / High-Yield Fact**
BSE is a zoonotic disease, meaning it can be transmitted from animals to humans, and it has been linked to a human variant of the disease called variant Creutzfeldt-Jakob disease (vCJD). The risk of transmission is thought to be associated with the consumption of contaminated beef products.

**Correct Answer:** C. Prion protein (PrP) in cattle.