In which part of the body do adult Schistosoma worms reside?

Adult Schistosoma worms predominantly reside in the blood vessels of the host. Specifically, they are usually found in the mesenteric veins that drain the intestines or the pelvic veins. The mature worms live within these blood vessels, where they can feed on the host's nutrients and reproduce. This location is crucial for their lifecycle, as it allows them to release eggs, which can then be transported through the bloodstream and eventually exit the host through excretion. This characteristic habitat is a key factor in the pathology associated with schistosomiasis, as the presence of the worms and their eggs in the blood vessels can lead to inflammation and fibrosis of the surrounding tissues.

In contrast, the other regions mentioned are not suitable habitats for adult Schistosoma worms. The intestines typically host different types of parasites and are not the primary residence for Schistosoma. The liver does play a role in the immune response against the eggs produced by the worms but is not where the adult worms themselves reside. Muscles are similarly not involved in the life cycle of Schistosoma. Thus, the blood vessels of the host are the correct answer, as they directly accommodate the adult worms and are integral to the transmission and pathology of schistosomiasis.