All of the following are RNA viruses except for which one?

Hepatitis B virus (HBV) is the exception in this question as it is a DNA virus, specifically a member of the Hepadnaviridae family. Unlike RNA viruses, which utilize ribonucleic acid as their genetic material, HBV has deoxyribonucleic acid (DNA) as its genome.

In contrast, the other viruses listed — hepatitis A virus (HAV), hepatitis C virus (HCV), and hepatitis D virus (HDV) — all contain RNA as their genetic material. HAV is a picornavirus, HCV is a flavivirus, and HDV is a unique virus that requires the presence of HBV for its replication but is classified as an RNA virus.

Understanding the difference between RNA and DNA viruses is crucial in microbiology and immunology, particularly when considering treatment options and vaccine development.