This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: Virology, Subject name: Microbiology
Mad Cow Disease has been highly publicized in Great Britain. This disease, which is similar to scrapie, is caused by 
Select and explain. 
A. A prion, B. A virus, C. Rickettsiae, D. An autoimmune reaction
A: A prion
Answer: A: A prion 
Explanation: Mad Cow Disease is related to both scrapie in sheep and bovine spongiform encephalopathy virus. The fear in Great Britain is the potential for acquiring Creutzfeldt-Jakob disease, which is a slowly progressive neurodegenerative disease. Theoretically, such acquisition could be through ingestion of beef from infected cows. A prion consists of protein material without nucleic acid. While related to a virus, a prion is a proteinaceous infectious particle that replicates within cells.