**Core Concept**
The filarial stage of adult worms responsible for diseases in humans is typically associated with the presence of microfilariae in the bloodstream. However, adult worms themselves can also cause disease, particularly when they migrate through tissues and cause mechanical damage.

**Why the Correct Answer is Right**
The correct answer is the stage of the filarial worm that is not typically associated with causing disease in humans. Adult filarial worms, such as those responsible for lymphatic filariasis (Wuchereria bancrofti and Brugia malayi) and onchocerciasis (Onchocerca volvulus), can cause significant morbidity due to their migration through tissues and the immune response they elicit. In contrast, the third-stage larvae of these worms are responsible for causing disease.

**Why Each Wrong Option is Incorrect**
**Option A:** Microfilariae are the stage of the filarial worm that is typically associated with causing disease in humans, as they circulate in the bloodstream and can cause allergic reactions and other symptoms.
**Option B:** Third-stage larvae are the stage of the filarial worm that is responsible for causing disease in humans, as they migrate through tissues and cause mechanical damage.
**Option C:** Adult filarial worms are responsible for causing disease in humans, particularly due to their migration through tissues and the immune response they elicit.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the clinical presentation of filarial disease can vary depending on the stage of the worm and the species involved. Microfilariae, third-stage larvae, and adult worms can all cause different symptoms and complications, making accurate diagnosis and treatment critical.

**Correct Answer:** D.