How does poliovirus reach the central nervous system after gastrointestinal replication?

• A. Viremia allows spread to CNS via blood-brain barrier or retrograde axonal transport
• B. Direct invasion from gut to brain via olfactory nerves
• C. Poliovirus cannot reach CNS
• D. Through lymphatic drainage solely

Answer: (A)

After poliovirus replicates in the gut, the virus can enter the bloodstream (viremia), allowing it to disseminate to distant sites, including the central nervous system. From there, it can reach motor neurons in the spinal cord and brainstem and cause paralysis. The CNS invasion occurs either by the virus crossing the blood–brain barrier during viremia or by traveling along peripheral nerves in a retrograde fashion toward the CNS. A direct gut-to-brain route via olfactory nerves is not the typical path for poliovirus, and lymphatic drainage alone does not account for CNS invasion. The essential idea is that systemic spread through the bloodstream after GI replication opens the route to the CNS, with nerve-based transport as a possible additional mechanism.