A 4-year-old unvaccinated child from rural West Bengal presents with a 2-week history of fever and malaise, followed by sudden onset of weakness in the right lower limb. On examination, the child has **asymmetric flaccid paralysis** (marked **A** in the diagram) with absent deep tendon reflexes in the affected limb and preserved sensation. Stool samples are sent for viral isolation. Which of the following is the most likely causative organism responsible for this clinical presentation?

Explanation

## Why Poliovirus (enterovirus, serotypes 1, 2, or 3) is correct The clinical presentation of **asymmetric flaccid paralysis** (marked **A**) in a previously well child with preserved sensation and areflexia is the cardinal sign of acute paralytic poliomyelitis. Poliovirus is an enterovirus with three serotypes (1, 2, 3) that selectively destroys anterior horn motor neurons in the spinal cord, producing lower motor neuron signs (flaccidity, areflexia, hypotonia). The biphasic illness pattern (prodrome → recovery → recurrence of paralysis at days 9–14) and asymmetric limb involvement (commonly affecting one leg) are pathognomonic. Stool viral isolation and serology confirm the diagnosis. India was certified polio-free in 2014, but unvaccinated children remain at risk if exposed to wild-type virus from endemic regions (Afghanistan, Pakistan) or vaccine-derived poliovirus in undervaccinated populations (Nelson 21e Ch 277; Park 26e). ## Why each distractor is wrong - **Coxsackievirus B**: While an enterovirus, Coxsackievirus B causes myositis and typically presents with symmetric ascending paralysis and demyelination (more consistent with GBS), not the selective anterior horn cell destruction and asymmetric pattern seen in polio. - **Guillain-Barré syndrome**: GBS presents with symmetric ascending paralysis (starts distally in legs, ascends), elevated CSF protein with normal cell count, and demyelination on nerve conduction studies. It does NOT produce the asymmetric, selective anterior horn cell pattern or the biphasic illness seen in polio. - **Transverse myelitis**: Presents with a sensory level (preserved sensation above, lost below), bowel/bladder involvement, and bilateral signs. The preserved sensation without a sensory level in this case rules out myelopathy. **High-Yield:** Asymmetric flaccid paralysis with preserved sensation and areflexia = polio until proven otherwise; always investigate acute flaccid paralysis (AFP) in children <15 years with stool samples and serology. [cite: Nelson 21e Ch 277; Park 26e]