This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: All India exam, Subject name: Pharmacology
"Mad cow" disease is caused by? (NOT RELATED) 
Select and explain. 
A. HIV virus, B. Mycoplasma, C. Chlamydia, D. Prions
D: Prions
Answer: D: Prions 
Explanation: Several animal diseases from this group are also known, including scrapie in sheep and goats and bovine spongiform encephalopathy in cattle ("mad cow" disease).All these disorders are associated with abnormal forms of a normal cellular protein, termed prion protein (PrPc).