Spongiform encephalopathy is caused by

Bovine spongiform encephalopathy (BSE) or mad cow disease is a neurodegenerative disease in cattle that may be passed to humans who have eaten infected flesh. It is caused by a specific type of misfolded protein called a prion. Prions replicate by causing other normally folded proteins to misfold and leading to an exponential chain reaction. Then the prions aggregate into an alpha helical and beta pleated sheet which is thought to be toxic to brain cells and causes symptoms. So, the correct answer is 'Prion'.