**Question:** Site of action of polio virus is

A. Motor neurons
B. Central nervous system
C. Gastrointestinal tract
D. Respiratory system

**Core Concept:** Poliomyelitis, caused by the polio virus, is an infectious disease that primarily affects the motor neurons in the central nervous system (CNS) and gastrointestinal tract (GIT). The virus targets and damages these specific cells, leading to the characteristic symptoms and complications associated with the illness.

**Why the Correct Answer is Right:** Polio virus primarily targets motor neurons in the spinal cord and brainstem, which are responsible for controlling voluntary muscle movements. By damaging these neurons, the virus causes muscle weakness, paralysis, and in severe cases, respiratory failure. Additionally, the virus can also infect intestinal enterocytes, leading to gastrointestinal symptoms such as diarrhoea and vomiting.

**Why Each Wrong Option is Incorrect:**

A. Motor neurons are indeed affected by the polio virus, but the primary site of action is the central nervous system (CNS), not just motor neurons.
B. Although the virus can cause inflammation and damage to the CNS, the correct answer focuses on the site of viral entry and replication, which is the gastrointestinal tract (GIT).
C. The gastrointestinal tract is where the polio virus replicates and spreads, but the primary site of action, causing motor neuron damage and paralysis, is the central nervous system (CNS).
D. The respiratory system is not the primary site of action for polio virus; rather, it is the CNS that is affected, leading to the neurological symptoms and complications.

**Clinical Pearl:** Polio virus is an example of a neurotropic virus, meaning it targets and affects neurons. Understanding the site of action of the virus is crucial for understanding the disease manifestations, diagnosis, and vaccination strategies against polio. The site of action explains the neurological and gastrointestinal symptoms associated with poliomyelitis, and highlights the importance of the Salk and Sabin polio vaccines, which target the virus in the GIT to prevent neuronal infection and subsequent paralysis.