## **Core Concept**
The question describes a clinical presentation suggestive of a parasitic infection, specifically one that causes subcutaneous nodules. The presence of microfilariae and adult worms in skin scrapings points towards a filarial infection.

## **Why the Correct Answer is Right**
The correct answer, **D. Onchocerca volvulus**, is right because Onchocerca volvulus is a parasitic worm that causes onchocerciasis, also known as river blindness. A key feature of this infection is the formation of subcutaneous nodules (onchocercomata) that contain adult worms. The skin is also a key site for the microfilariae, which are the larval stages of the worm. These microfilariae can be found in skin scrapings, making the description in the question highly suggestive of onchocerciasis.

## **Why Each Wrong Option is Incorrect**
- **Option A:** *Wuchereria bancrofti* is incorrect because it primarily causes lymphatic filariasis, characterized by lymphedema and elephantiasis, not subcutaneous nodules.
- **Option B:** *Brugia malayi* is incorrect for similar reasons as *Wuchereria bancrofti*; it causes lymphatic filariasis and is less commonly associated with subcutaneous manifestations.
- **Option C:** *Mansonella* species can cause filariasis but are less commonly associated with the formation of subcutaneous nodules and the specific presentation described.

## **Clinical Pearl / High-Yield Fact**
A key clinical pearl is that onchocerciasis is not only identified by subcutaneous nodules but also by skin and eye lesions. The microfilariae can cause severe dermatitis and are also known to affect the eyes, potentially leading to blindness. The presence of "hanging groin" or severe skin and eye involvement should raise suspicion for this diagnosis.

## **Correct Answer:** **D. Onchocerca volvulus**