Answer:
a. Tropism
Step-by-step explanation:
The entry of pathogens into the host cell requires attachment of the specific proteins of the pathogen to the receptors of the host cell. Various tissues of a host organism have a differential distribution of the host cell receptors to which the pathogen or viruses attach. This regulates the tropism which is the feature of a particular pathogen to infect a specific tissue type. For example, the receptors for the Poliovirus are found only in the human nasopharynx, gut, and cells of the anterior horn of the spinal cord. Therefore Poliovirus infects these tissues.