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?

**Core Concept**
The question is testing the student's knowledge of a specific intestinal parasite that causes gastrointestinal symptoms in children. The organism in question is a protozoan that has both a trophozoite and cyst form, and is commonly found in contaminated water or food.

**Why the Correct Answer is Right**
The organism described is *Giardia lamblia*, a flagellated protozoan that infects the small intestine and causes giardiasis. The pear-shaped trophozoites are motile and have four pairs of flagella, which help them to attach to the intestinal mucosa and absorb nutrients. The cyst form is highly resistant to environmental stress and can survive for weeks outside the host. The symptoms of giardiasis, including loose bulky stools and abdominal pain, are due to the trophozoites' ability to cause malabsorption and inflammation in the small intestine.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because *Entamoeba histolytica* has a larger trophozoite with a clear halo around the nucleus, and cysts with four nuclei are not characteristic of this species.
* **Option B:** This option is incorrect because *Balantidium coli* is a ciliated protozoan that is typically found in the large intestine and causes a different set of symptoms.
* **Option C:** This option is incorrect because *Cryptosporidium parvum* is a coccidian parasite that causes a different set of symptoms, including watery diarrhea and weight loss.

**Clinical Pearl / High-Yield Fact**
The key to diagnosing giardiasis is to look for the characteristic pear-shaped trophozoites with four pairs of flagella in the stool examination. Treatment typically involves metronidazole or albendazole, and patients should also be advised to avoid contaminated water and food sources to prevent reinfection.

**Correct Answer:** D.

✓ Correct Answer: B. Giardia lamblia