Defective satellite viruses in hepatitis – Identify the agent that requires hepatitis B virus (HBV) for its replication and assembly (envelope supply).

Introduction:
Among the human hepatitis viruses, one agent is unique because it cannot complete its life cycle without help from another virus. This question tests your understanding of the defective satellite nature of hepatitis D virus (HDV) and why it depends on hepatitis B virus (HBV) for infectious particle formation.

Given Data / Assumptions:

• HDV possesses a circular single-stranded RNA genome and delta antigen (HDAg).
• HBV provides the envelope protein (HBsAg) needed for HDV virion assembly and egress.
• HDV can infect as coinfection with HBV or as superinfection in an existing HBV carrier.

Concept / Approach:
Classify hepatitis viruses by structure and replication requirements. Identify which agent is a satellite virus: one that uses another virus's proteins to package its genome and exit cells. Determine which hepatitis virus supplies the missing structural components for HDV.

Step-by-Step Solution:
Step 1: Recall that HDV lacks its own envelope glycoproteins and requires HBsAg from HBV.Step 2: Recognize that without concurrent HBV infection, HDV cannot assemble infectious virions.Step 3: Distinguish HDV from HAV, HCV, and HEV, which are independent viruses with their own assembly machinery.Step 4: Conclude that the defective satellite virus among the options is HDV.

Verification / Alternative check:
Clinical patterns support this biology: HDV coinfection causes acute hepatitis with HBV, while HDV superinfection of an HBsAg carrier often leads to severe disease because HBV supplies abundant HBsAg for HDV packaging.

Why Other Options Are Wrong:

• HBV: a complete DNA virus; not defective.
• HCV: an RNA flavivirus; independent replication.
• HAV/HEV: enterically transmitted RNA viruses; no dependency on HBV.

Common Pitfalls:
Assuming all hepatitis viruses behave similarly; overlooking the requirement for HBsAg in HDV assembly.

Final Answer:
Hepatitis D virus (HDV).