Which of the following organisms is not a protozoan?

Step 1: Analyze the question We need the organism that is not a protozoan. Protozoa are single-celled (unicellular) eukaryotic organisms, usually microscopic.

Step 2: Explain the concept Protozoa are classified by how they move:

• Amoeboids (pseudopodia): Amoeba
• Ciliates (cilia): Paramecium
• Flagellates (flagella): Trypanosoma
• Sporozoans (non-motile, parasitic): Plasmodium

Step 3: Check each option

• A. Amoeba: unicellular, moves by pseudopodia, so it is a protozoan.
• B. Ascaris: this is the roundworm, a multicellular parasite in the phylum Nematoda. It is not a protozoan.
• C. Plasmodium: the unicellular parasite that causes malaria, so it is a protozoan.
• D. Paramecium: a unicellular ciliate, so it is a protozoan.

Step 4: Answer The correct option is B. Ascaris. It is a multicellular worm (Nematoda), while the others are all single-celled protozoans.

Tip: Remember that "Ascaris = worm." Anything ending in "-worm" or belonging to Nematoda, Platyhelminthes, or Annelida is multicellular. For protozoa, think of the four "movers": Amoeba (pseudopodia), Paramecium (cilia), Trypanosoma (flagella), and Plasmodium (non-motile in the adult stage). Nigerian students often remember malaria = Plasmodium = protozoan.