Final answer:
In a direct saline fecal smear, observation of a motile, binucleate trophozoite with flagella is indicative of Giardia lamblia, which causes giardiasis.
Step-by-step explanation:
Observation of a motile, flagellate, binucleate trophozoite in a direct saline fecal smear is indicative of Giardia. Giardia lamblia (also known as Giardia intestinalis) is a flagellate protozoan that causes giardiasis, which is characterized by symptoms such as diarrhea, abdominal pain, and fever. It attaches to the lining of the small intestine, hindering nutrient absorption. Diagnosis of Giardia can involve direct microscopic examination of stool samples, immunoassays, or fluorescence microscopy, where it can exhibit a characteristic appearance.
Detection of Ancylostoma, which are hookworms, would typically involve the identification of eggs rather than trophozoites. Isospora and Cryptosporidium infections also exhibit differing morphological features under microscopic examination when compared to Giardia.