Which parasite stage is immediately infective for humans on passage from the gastrointestinal tract?

The correct choice is based on the understanding of the life cycle of Cryptosporidium spp. and how its oocysts interact with human hosts. Cryptosporidium spp. is a protozoan parasite that causes gastrointestinal illness. The oocysts are the infective stage of this parasite and are excreted in the feces of infected individuals. These oocysts are highly resistant to environmental factors and can survive for long periods outside the host.

Once a person ingests these oocysts, typically through contaminated water or food, they can immediately infect the gastrointestinal tract and develop into the active form, leading to an infection known as cryptosporidiosis. The emphasis on the oocysts being immediately infective upon ingestion distinguishes them from other life stages of different parasites, which may require further development or transformation before becoming infectious to humans.

In contrast, the stages associated with the other options involve additional steps before they can lead to an infection in humans. For example, Schistosoma spp. typically require a fresh-water intermediate host to develop into the cercarial stage before infecting humans. Toxoplasma gondii bradyzoites are encysted forms that don’t cause infection until they are released from the cysts