Which organism allows humans to serve as both the intermediate and definitive host?

The organism that allows humans to serve as both the intermediate and definitive host is Hymenolepis nana, commonly known as the dwarf tapeworm. This tapeworm is unique because it can complete its life cycle within a single host, which in this case is humans.

In a typical life cycle, a definitive host is where the adult form of the parasite lives and reproduces, while an intermediate host typically harbors the larval stage. Hymenolepis nana does not require an intermediate host in environments with suitable conditions and can develop directly in humans, where it can also produce eggs. This ability contributes to its transmission and prevalence in humans, particularly in environments with poor sanitation.

This characteristic sets Hymenolepis nana apart from other options provided. For instance, other organisms mentioned, such as Schistosoma japonicum and Ascaris lumbricoides, rely on specific intermediate hosts to complete their life cycles. Enterobius vermicularis, while having a straightforward life cycle involving humans, functions differently, with humans only serving as the definitive host. Understanding the life cycles and host relationships of these organisms is crucial in microbiology and parasitology.