**Core Concept**
The **Trichuris trichiura**, or whipworm, is a parasitic nematode that infects the human large intestine. The life cycle of *T. trichiura* involves the ingestion of fully embryonated eggs.

**Why the Correct Answer is Right**
The fully embryonated egg of *Trichuris trichiura* contains a **second-stage larva**, which is the infective stage of the parasite. When ingested, the eggshell is broken down in the intestine, releasing the larva, which then molts and develops into an adult worm.

**Why Each Wrong Option is Incorrect**
**Option A:** This choice is incorrect because the first-stage larva is not the stage typically found in the fully embryonated egg.
**Option B:** This choice is incorrect as it does not accurately describe the contents of a fully embryonated *T. trichiura* egg.
**Option D:** This choice is also incorrect as the fully embryonated egg does not contain an adult worm.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that *T. trichiura* eggs are highly resistant to environmental factors and can survive for extended periods outside of a host, making them a significant source of infection.

**Correct Answer:** D. Second stage larva