Final answer:
In the case of an occult heartworm infection, there are no microfilariae present in the host's bloodstream; the answer is (a) Microfilariae.
Step-by-step explanation:
An occult heartworm infection means there are no microfilariae present in the definitive host. The correct answer to the student's question is (a) Microfilariae. In the context of heartworm infections, adult heartworms normally reside in the right ventricle of the heart in dogs and in the pulmonary arteries of cats, where they produce microfilariae that can be seen in blood smears. However, when there is an occult infection, the adult worms may be present, yet the microfilariae are not detectable in the bloodstream. Conditions like this can occur for various reasons such as the immune system clearing the microfilariae or the presence of only male worms that can't produce offspring.