**Core Concept:** Filariasis is a mosquito-borne parasitic infection caused by filarial worms, belonging to the genus Wuchereria, Brugia, or Onchocerca. The life cycle involves both a mammalian host and a vector (mosquito).

**Why the Correct Answer is Right:** The correct answer, **D. The life cycle of filaria involves two hosts and an intermediate vector**. Filariasis is caused by filarial worms, which have a complex life cycle involving two hosts and an intermediate vector. In the life cycle of filarial worms, the adult worms live in the blood vessels of the mammalian host, while the larvae (microfilariae) circulate in the blood, where they are taken up by the vector mosquito during blood meal. Inside the mosquito, the larvae mature into infective larvae (L3 or L4) within the mosquito body, and then these larvae are transmitted to the mammalian host when the mosquito bites and injects the larvae into the host's skin during blood meal.

**Why Each Wrong Option is Incorrect:**

A. **Wrong Option A: The life cycle only involves the mammalian host**. This choice is incorrect because filarial worms have two hosts in their life cycle - a mammalian host and a vector (mosquito).

B. **Wrong Option B: The life cycle occurs within the mosquito only**. This choice is incorrect because the life cycle involves both the mammalian host and the mosquito. The larvae mature within the mosquito and are transmitted to the mammalian host.

C. **Wrong Option C: The life cycle is direct and does not involve a vector**. This choice is incorrect because filarial worms require a vector (mosquito) to complete their life cycle. The larvae mature within the mosquito and are transmitted to the mammalian host during the mosquito's blood meal.

D. **Correct Option D: The life cycle involves two hosts and a vector**. This choice is correct as it accurately describes the two-hosts and vector life cycle of filarial worms.

**Clinical Pearls:**

1. **Clinical Pearls (1-3)**:
- Understanding the life cycle of filarial worms is crucial for diagnosing and treating filariasis.
- Knowledge of the vector (mosquito) is essential for preventive measures, such as insecticide-treated bed nets, indoor residual spraying, and larvicides.
- Transmission of filarial worms is seasonal, especially in endemic areas, as the mosquito vector is active during the day and night.

Please note that the explanation provided here is a simplified version of the life cycle, which includes more stages and hosts (e.g., anopheles mosquito for Wuchereria bancrofti and culicine mosquito for Brugia malayi and Brugia timori). However, the given question does not specify the type of filarial worm, so the explanation provided is applicable to all three species mentioned.