This is a real-world medical entrance exam question, please give the true answer based on the question and selection. Topic name: parasitology, Subject name: Microbiology
A 4 year old child presents with acute watery diarrhea and abdominal cramps. Stool microscopy reveals trophozoites with falling leaf motility. The etiological agent is? 
Select and explain. 
A. Entamoeba hisiolutics, B. Giardia lamblia, C. Trichomonas tenax, D. Balantidium coli
B: Giardia lamblia
Answer: B: Giardia lamblia 
Explanation: The trophozoite is motile, with a slow oscillation about its long axis, which has been likened to the motion of a 'falling leaf.' It divides by longitudinal binary fission. It lives in the duodenum and upper pa of the jejunum attached by means of the sucking disc to the epithelial cells of the villi and crypts feeding by pinocytosis