**Core Concept**
Visceral larva migrans is a clinical condition caused by the migration of larvae of certain nematodes through various tissues, leading to eosinophilia and organ enlargement. The causative organisms are typically parasites that have a lifecycle involving a definitive host and an intermediate host.

**Why the Correct Answer is Right**
The causative organism of visceral larva migrans is a nematode that has a lifecycle involving a definitive host, usually a carnivore or omnivore, and an intermediate host, usually a mammal or bird. In this case, the dog trainer is likely the intermediate host. The nematode larvae are ingested through contaminated food or water, and upon reaching the gastrointestinal tract, they penetrate the intestinal wall and migrate to various organs, including the liver and spleen. The eosinophilia is a result of the host's immune response to the migrating larvae.

**Why Each Wrong Option is Incorrect**
**Option A:** Toxocara cati is a nematode that primarily infects cats, and while it can cause visceral larva migrans in humans, it is not the most common causative organism.
**Option B:** Toxocara canis is the most common causative organism of visceral larva migrans in humans, as it is a common parasite of dogs and can easily be transmitted to humans through contaminated soil or food.
**Option C:** Anisakis is a nematode that primarily infects fish and can cause anisakiasis in humans, but it is not typically associated with visceral larva migrans.

**Clinical Pearl / High-Yield Fact**
Visceral larva migrans can occur in individuals who have not traveled to endemic areas, as the parasite can be transmitted through contaminated food or soil in urban areas.

**Correct Answer: B. Toxocara canis**