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
Hepatitis D virus (delta agent) is a defective virus that can replicate only in cells already infected with which of the following viruses? 
Select and explain. 
A. Hepatitis A virus, B. Epstein-Barr virus, C. Hepatitis G virus, D. Hepatitis B virus
D: Hepatitis B virus
Answer: D: Hepatitis B virus 
Explanation: Hepatitis D virus is a defective virus with an RNA genome and a hepatitis B surface antigen envelope. Two types of HDV infection occur: simultaneous HDV and HBV infection, or HDV superinfection with chronic HBV infection. Diagnosis is made by demonstrating IgM or IgG antibodies, or both.