Which of the following is the infectious stage of a protozoan?
A Spore
B Cyst
C Trophozoite
D Seed
The correct answer and explanation is:
The correct answer is C) Trophozoite.
Explanation:
Protozoans, which are single-celled organisms belonging to the kingdom Protista, have complex life cycles that often involve different stages, each adapted for survival in specific environmental conditions. Among these stages, the trophozoite is typically the active, feeding, and motile form of the protozoan, making it the infectious stage in most cases.
The trophozoite stage is where the protozoan is actively growing and reproducing. It is the form that typically invades host tissues, causing infection. For example, in diseases like amoebic dysentery caused by Entamoeba histolytica, the trophozoite is the form that invades the intestines and causes symptoms.
In contrast, the spore (A) is usually a resistant, dormant stage that helps the organism survive harsh conditions, but it is not actively involved in infection. Spores are formed by certain protozoans like Plasmodium (the causative agent of malaria) during their life cycle and are typically spread through vectors like mosquitoes. They are resistant to environmental stress and are not responsible for causing the active infection.
The cyst (B) is another dormant stage. It is a protective, tough shell that the protozoan forms to survive in harsh conditions outside of a host. While cysts can be ingested by a host, they must mature into trophozoites inside the host to cause disease.
The seed (D) is not a stage found in protozoans. Seeds are associated with plants and are not relevant to protozoan life cycles.
In summary, the trophozoite is the stage that actively interacts with the host and is responsible for the infection.