A person returns to Delhi from Bangladesh after 2 days and has diarrhoea. Stool examination shows RBC&;s in the stool. The likely organism is
**Core Concept**
The question is testing the ability to identify a causative organism of a specific gastrointestinal infection, particularly in a traveler returning from a region with a high prevalence of certain pathogens.
**Why the Correct Answer is Right**
The presence of RBCs in the stool suggests a bloody diarrhea, which is characteristic of invasive pathogens that cause intestinal inflammation. The incubation period of 2 days also points towards a bacterial cause. Among the options, **Shigella dysenteriae** is a classic example of an invasive bacterium that causes dysentery, characterized by bloody diarrhea and RBCs in the stool. Shigella dysenteriae secretes Shiga toxin, which causes intestinal epithelial cell damage and inflammation, leading to the presence of RBCs in the stool.
**Why Each Wrong Option is Incorrect**
**Option A:** Vibrio cholerae is a gram-negative bacterium that causes cholera, characterized by profuse watery diarrhea without blood. It is not associated with the presence of RBCs in the stool.
**Option B:** Giardia lamblia is an intestinal protozoan parasite that causes giardiasis, characterized by watery diarrhea without blood. It is not associated with the presence of RBCs in the stool.
**Option C:** Cryptosporidium parvum is an intestinal protozoan parasite that causes cryptosporidiosis, characterized by watery diarrhea without blood. It is not associated with the presence of RBCs in the stool.
**Clinical Pearl / High-Yield Fact**
When evaluating a patient with bloody diarrhea, it is essential to consider the epidemiological history, including travel history, to narrow down the differential diagnosis. In this case, the patient's recent travel to Bangladesh increases the likelihood of Shigella dysenteriae infection.
**Correct Answer:** C. Shigella dysenteriae.