**Core Concept**
The question is testing the student's knowledge of a specific parasitic infection that can cause a duodenal stricture. Duodenal stricture is a narrowing of the duodenum, the first part of the small intestine, which can cause obstructive symptoms. This condition can be caused by various factors, including parasitic infections.

**Why the Correct Answer is Right**
The correct answer is **A. Fasciola hepatica**. Fasciola hepatica, also known as the liver fluke, is a parasitic flatworm that infects the liver and bile ducts. However, during its life cycle, it can migrate through the duodenum and cause inflammation, leading to fibrosis and subsequent stricture formation. This is a well-documented complication of fascioliasis.

**Why Each Wrong Option is Incorrect**
**Option B:** *Toxocara spp.* is a group of parasitic roundworms that primarily infect the lungs and intestines but are not typically associated with duodenal stricture.

**Option C:** *Entamoeba histolytica* is a protozoan parasite that causes amoebiasis, primarily affecting the large intestine. While it can cause intestinal ulcers and perforation, duodenal stricture is not a typical complication.

**Option D:** *Opisthorchis viverrini* is a liver fluke similar to F. hepatica but is primarily associated with cholangiocarcinoma and bile duct strictures rather than duodenal strictures.

**Clinical Pearl / High-Yield Fact**
Fasciola hepatica infection can be diagnosed through serological tests or fecal examination. It is essential to consider this parasite in the differential diagnosis of duodenal strictures, particularly in patients with a history of exposure to contaminated water sources or handling infected animals.

**Correct Answer:** A. Fasciola hepatica.