Which of the following is a medically important protozoan?

• A. Giardia lamblia
• B. Plasmodium falciparum
• C. Entamoeba histolytica
• D. Toxoplasma gondii

Answer: (A)

Protozoan parasites that invade humans are a major focus in clinical parasitology, and recognizing those you’re most likely to encounter in practice is essential. Giardia lamblia is a leading example of a medically important intestinal protozoan. It forms cysts in the environment and trophozoites in the small intestine, and these forms drive transmission and infection. The trophozoite’s distinctive appearance—pear-shaped with two nuclei and multiple flagella, plus an adhesive ventral disc that lets it cling to the intestinal lining—helps explain both its pathogenicity and how it is detected in the lab. Clinically it causes giardiasis, a common diarrheal illness that can be acquired from contaminated water or food, leading to malabsorption and fatty stools. Because Giardia is routinely targeted in diagnostic stool testing and is a textbook example of an intestinal protozoan parasite, it is considered a medically important protozoan in this context.