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
Delta hepatitis only occurs in patients who also have either acute or chronic infection with hepatitis B virus. The delta agent is 
Select and explain. 
A. An incomplete hepatitis B virus, B. Related to hepatitis A virus, C. A hepatitis B mutant, D. An incomplete RNA virus
D: An incomplete RNA virus
Answer: D: An incomplete RNA virus 
Explanation: The delta agent was first described in 1977 and has recently been shown to be an incomplete RNA virus that requires HBsAg for replication. It is found most often in persons who have multiple parenteral exposures, for example, intravenous (IV) drug abusers, hemophiliacs, and multiply transfused patients.