Virus morphology — Enveloped viruses typically exhibit what overall particle shape under electron microscopy?

Introduction:
Viral morphology can be icosahedral, helical, complex, or pleomorphic. Envelopes are host-derived lipid bilayers surrounding the nucleocapsid and often lead to variability in shape. This question examines recognition of envelope-associated morphology.

Given Data / Assumptions:

• Enveloped particles acquire membranes by budding.
• Membrane flexibility allows size and shape variability.
• Internal symmetry can be helical or icosahedral even when the outer shape appears pleomorphic.

Concept / Approach:
Because the lipid envelope is deformable, many enveloped viruses look spherical or pleomorphic rather than strictly geometric. Classic examples include influenza (pleomorphic), togaviruses (spherical), and coronaviruses (spherical with surface projections).

Step-by-Step Solution:

Verification / Alternative check:
EM atlases consistently depict many enveloped virions as spheres with variable diameters or irregular outlines, unlike rigid nonenveloped icosahedral capsids.

Why Other Options Are Wrong:

• Icosahedral: common for nonenveloped capsids; envelope masks strict geometry.
• Helical: describes nucleocapsid symmetry; the envelope often yields pleomorphism.
• Complex: typical for tailed bacteriophages, not enveloped animal viruses.

Common Pitfalls:
Conflating internal symmetry with overall particle outline; an enveloped virion can contain an icosahedral core but appear spherical externally.

Final Answer:
Roughly spherical or pleomorphic