**Core Concept**
Mad Cow Disease, also known as Bovine Spongiform Encephalopathy (BSE), is a neurodegenerative disorder that affects cattle. It is caused by an infectious agent that is distinct from traditional viruses and bacteria. The disease shares similarities with scrapie, a prion disease affecting sheep.

**Why the Correct Answer is Right**
The correct answer is a prion, which is a misfolded protein that can cause normal proteins in the brain to also misfold. Prions are unique in that they can replicate without nucleic acids, such as DNA or RNA. In the case of BSE, the prion protein (PrP^Sc) is transmitted through contaminated animal feed, leading to brain degeneration and death. The prion theory of disease was first proposed by Stanley Prusiner, who was awarded the Nobel Prize in Physiology or Medicine in 1997 for his work.

**Why Each Wrong Option is Incorrect**
**Option B:** A virus is a type of infectious agent that contains nucleic acids and is not responsible for causing Mad Cow Disease. Viruses are typically treated with antiviral medications or vaccines, which are not effective against prion diseases.
**Option C:** Rickettsiae are a type of bacteria that cause diseases such as typhus and spotted fever, but they are not associated with Mad Cow Disease. Bacteria are typically treated with antibiotics, which are not effective against prion diseases.
**Option D:** An autoimmune reaction occurs when the body's immune system attacks its own tissues, but it is not the cause of Mad Cow Disease. Autoimmune reactions are typically treated with immunosuppressive medications, which are not effective against prion diseases.

**Clinical Pearl / High-Yield Fact**
It's essential to note that prion diseases are transmissible to humans through contaminated animal products, leading to variant Creutzfeldt-Jakob disease (vCJD). This highlights the importance of proper meat handling and slaughter practices to prevent the spread of prion diseases.

**✓ Correct Answer: A. A prion**