Final answer:
A microbial pathogen that benefits at the expense of its host is considered a parasite, indicative of a parasitic relationship where the host is harmed by the parasite's actions.
Step-by-step explanation:
A microbial pathogen that derives benefits from its host and harms the host in the process is considered a parasite. This defines a symbiotic relationship known as parasitism, where the parasite species lives in or on another species, called the host, and causes harm to the host while benefitting from this relationship. Parasitism typically involves the parasite weakening the host by consuming its resources but not necessarily killing it quickly, as the parasite often has a complex reproductive cycle that requires the host to survive long enough to allow the spread to additional hosts.
In contrast, commensalism refers to a relationship where one organism benefits and the other is unaffected, and is therefore not the correct answer to this question. Since the pathogen in question harms the host, we can rule out mutualism, where both species benefit, and commensalism, where one benefits without affecting the other. Consequently, the correct answer is A. Parasite.