Which of the following is the only common pathogenic protozoan found in the duodenum and jejunum?

Giardia lamblia (also referred to as Giardia duodenalis or Giardia intestinalis) is the causative agent of giardiasis and is the only common pathogenic protozoan found in the duodenum and jejunum of humans. Large numbers of parasites attached to the bowel wall may cause irritation and low-grade inflammation of the duodenal or jejunal mucosa, with consequent acute or chronic diarrhea associated with crypt hyperophy, villous atrophy or flattening, and epithelial cell damage. Stools may be watery, semisolid, greasy, bulky, and foul smelling at various times during the course of the infection. Ref: Brooks G.F., Carroll K.C., Butel J.S., Morse S.A., Mietzner T.A. (2013). Chapter 46. Medical Parasitology. In G.F. Brooks, K.C. Carroll, J.S. Butel, S.A. Morse, T.A. Mietzner (Eds), Jawetz, Melnick, & Adelberg's Medical Microbiology, 26e.