Infections caused by which organism allow humans to serve as both the intermediate and definitive host?

Hymenolepis nana, commonly known as the dwarf tapeworm, is the correct answer because it uniquely has a life cycle that involves humans as both intermediate and definitive hosts. In this species, humans can ingest eggs directly from contaminated food or water, which leads to the development of larvae that mature into adult worms in the intestines.

Unlike other parasites listed, Hymenolepis nana does not require an insect intermediate host for its lifecycle. It can complete its development entirely within humans, allowing them to harbor and propagate the infection. This characteristic distinguishes Hymenolepis nana from organisms like Enterobius vermicularis, Schistosoma japonicum, and Ascaris lumbricoides, each of which has distinct roles for definitive and intermediate hosts and typically requires additional hosts for their developmental stages. For example, Schistosoma japonicum relies on aquatic snails as intermediate hosts before returning to infect humans.

In light of this, Hymenolepis nana's ability to utilize humans in both roles highlights its unique life cycle and its significance in parasitology.