Measles virus cytopathic effect: In cell culture, infection with measles virus most characteristically leads to which observable effect?

Introduction:
Paramyxoviruses, including measles virus (Morbillivirus), express fusion proteins that can mediate cell–cell fusion. Recognizing the hallmark cytopathic effect (CPE) of measles infection in culture is important for laboratory diagnosis and understanding pathogenesis.

Given Data / Assumptions:

• Measles virus is an enveloped, negative-sense RNA virus of the Paramyxoviridae family.
• Viral fusion (F) protein on infected cell membranes can cause neighboring cells to fuse.
• Characteristic CPE includes formation of multinucleated giant cells (syncytia).

Concept / Approach:
Different viruses produce distinct CPEs: some cause rounding and lysis, others produce inclusion bodies, and some drive syncytium formation through fusion proteins. For measles, the most striking and testable feature is syncytium formation and, in tissues, Warthin–Finkeldey giant cells in lymphoid organs.

Step-by-Step Solution:
Step 1: Identify the virus family (Paramyxoviridae) and the presence of a fusion protein.Step 2: Understand that fusion of adjacent cell membranes yields multinucleated cells.Step 3: Recall that measles infection often shows cytoplasmic and sometimes nuclear inclusions, but the cardinal culture finding is syncytia.Step 4: Choose the option explicitly naming syncytium formation.

Verification / Alternative check:
Virology lab manuals describe syncytium formation as a rapid, recognizable CPE for measles and other paramyxoviruses (e.g., RSV). Histopathology texts note Warthin–Finkeldey giant cells in vivo, aligning with the in vitro fusion phenotype.

Why Other Options Are Wrong:

• Nuclear pyknosis: non-specific cell injury; not the distinctive hallmark.
• Neoplastic transformation: measles is not an oncogenic virus.
• Only rounding/aggregation: too generic; does not capture the fusion-driven giant cells.
• Intranuclear inclusions only: measles has cytoplasmic inclusions; the signature is syncytia rather than inclusions alone.

Common Pitfalls:
Confusing generic CPE (rounding, detachment) with the specific fusion-driven syncytia of paramyxoviruses.

Final Answer:
Syncytium (multinucleated giant cell) formation.