An anxious mother brought her 4 year old daughter to the pediatrician. The girl was passing loose bulky stools for the past 20 days. This was often associated with pain in abdomen. The pediatrician ordered the stool examination, which showed pear-shaped, flagellated trophozoites and cyst with four nuclei. What is the likely causative organism?

The typical morphology of the trophozoite and cyst of Giardia Lambia and the clinical picture make the diagnosis of Giardia Lamblia infection. Prominent early symptoms include diarrhea, abdominal pain, bloating, belching, flatus, nausea, and vomiting. Although diarrhea is common, upper intestinal manifestations such as nausea, vomiting, bloating, and abdominal pain may predominate. The duration of acute giardiasis is usually >1 week, although diarrhea often subsides. Ref: Weller P.F. (2012). Chapter 215. Protozoal Intestinal Infections and Trichomoniasis. In D.L. Longo, A.S. Fauci, D.L. Kasper, S.L. Hauser, J.L. Jameson, J. Loscalzo (Eds), Harrison's Principles of Internal Medicine, 18e.