<prompt> A 4-year-old child presents with acute watery diarrhea and abdominal cramps. Stool microscopy reveals trophozoites with falling leaf motility without RBCs and pus cells. The etiological agent is:Entamoeba histolyticaGiardia lambliaTrichomonas tenaxBalantidium coli  <response>: Ans. is b i.e., Giardia lambliaRef: D.R. Arora, p. 43 Mature cyst is the infective form of the parasite acquires infection by contaminated water and food. Infection may be acquired by anal oral sexual practices among male homosexuals. Within 30 minutes of ingestion excystation occurs in the duodenum, cyst hatches out two trophozoites which multiplies and colonize in duodenum and upper part of jejunum. In acute Giardiasis trophozoite show the typical falling-leaf motility in wet mount examination of faeces. 