The most frequently used portal of entry for pathogens is the

Mucosal surfaces of respiratory and gastrointestinal tract

Explanation:

A portal of entry is an avenue through which pathogenic microbes can have access to its host tissue. Since the pathogens are from the external environment, the portal of entry must be a site connected to the external.

Surfaces of the mucous membranes like respiratory tract, gastrointestinal tract etc. links the external and internal environment, hence, they are the most employed portals of entry by pathogens.

Also, the respiratory and gastrointestinal tract are vulnerable portals of entry because particles comprising of microbes are constantly taken in via the nose and mouth respectively.

high growth (apex)