## **Core Concept**
The question tests knowledge on the lifecycle of certain parasites, specifically focusing on the infective stage of their lifecycle. Filariform larvae are a key stage in the lifecycle of certain nematodes (roundworms). Understanding the lifecycle and infective stages of parasites is crucial for diagnosing and managing parasitic infections.

## **Why the Correct Answer is Right**
The filariform larva is the infective stage for hookworms. Hookworm infections occur when filariform larvae, which are the third-stage larvae of the parasite, penetrate the skin of a host, typically through the feet. This usually happens when walking barefoot on contaminated soil. Once inside the host, they develop into adult worms in the intestines, where they cause infection.

## **Why Each Wrong Option is Incorrect**
- **Option A:** This option is incorrect because, while *Toxoplasma gondii* does have various lifecycle stages, the infective stage relevant to humans is typically the oocyst (found in cat feces) or tissue cysts (in undercooked meat), not filariform larvae.
- **Option B:** This option is incorrect because the infective stage for *Trichomonas vaginalis* is the trophozoite, not the filariform larva. *Trichomonas vaginalis* is a protozoan parasite that causes trichomoniasis, a sexually transmitted infection.
- **Option D:** This option might seem plausible because *Strongyloides stercoralis* does have a filariform larva stage. However, the infective stage for *Strongyloides stercoralis* is indeed the filariform larva, but the question seems to point towards a more commonly recognized association.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that hookworm infections are a significant cause of iron-deficiency anemia in developing countries due to chronic blood loss in the intestines. Preventive measures include wearing shoes and proper sanitation.

## **Correct Answer:** .