Creutzfeldt-Jacob disease is caused by

A. prion.
B. bacilli.
C. cocci.
D. vibrio.

A. prion.

Creutzfeldt-Jacob disease is caused by A. prion.

Creutzfeldt-Jacob disease (CJD) is caused by A. prion.

To determine the cause of a disease like CJD, one can refer to credible medical sources, such as textbooks, scientific articles, or reliable healthcare websites. In this case, it is known that CJD is caused by a specific type of infectious agent called prions.

Prions are abnormal proteins that can fold into an altered shape, causing them to have a detrimental effect on brain function. When these misfolded prions accumulate in the brain, they can lead to the development of neurodegenerative disorders like Creutzfeldt-Jacob disease. Prions are highly resistant to standard sterilization techniques and can be transmitted through contaminated tissue or certain genetic mutations.

Therefore, the correct answer is A. prion.