HBV antigens – Identify which antigen is located in the viral envelope of hepatitis B virus.

Introduction:
Hepatitis B virus (HBV) particles contain distinct antigens that are used in diagnosis and indicate different stages of infection. Knowing which antigen resides in the envelope is critical for interpreting serology and understanding HDV dependency on HBV for envelope proteins.

Given Data / Assumptions:

• HBV structure includes an envelope, a nucleocapsid, and genomic DNA.
• Surface antigen (HBsAg) forms the envelope spikes and subviral particles.
• Core and e antigens are internal/secreted markers, not envelope spikes.

Concept / Approach:
Map antigens to virion components: the envelope is composed of lipids and HBsAg proteins (L, M, S). The core (HBcAg) encloses the genome and polymerase. HBeAg is a secreted derivative of the core/precore region, serving as a marker of replication, not a structural envelope component.

Step-by-Step Solution:
Step 1: Identify envelope proteins as HBsAg.Step 2: Exclude core-associated antigens (HBcAg, HBeAg) from the envelope.Step 3: Note HBx as a regulatory protein, not a major virion structural antigen.Step 4: Select HBsAg for the envelope.

Verification / Alternative check:
Laboratory detection of HBsAg indicates presence of HBV surface proteins in blood, matching its role as an envelope component and explaining formation of abundant noninfectious HBsAg spheres/filaments.

Why Other Options Are Wrong:

• HBcAg/HBeAg: nucleocapsid or secreted markers; not envelope proteins.
• HBxAg: regulatory; not a defining envelope antigen.
• Precore only: pertains to HBeAg synthesis; not envelope.

Common Pitfalls:
Equating abundant serum HBeAg with structural envelope location; misplacing HBx in the virion structure.

Final Answer:
HBsAg (surface antigen).